Yonago Acta medica 2003;46:91–102

The Clinical Aspects of β-Lactam-Resistant Stenotrophomonas maltophilia
Daisuke Kataoka and Yoshinori Tanaka
Division of Microbiology, Department of Microbiology and Pathology, School of Medicine,
Tottori University Faculty of Medicine, Yonago 683-8503 Japan
Recent papers argue that major increases in the isolation of multidrug-resistant Gramnegative bacteria have been of some concern in clinical practice. Among these bacteria,
Stenotrophomonas maltophilia is an intrinsic producer of β-lactamases, and has beenrecognized as a nosocomial pathogen. But few reports are available on the impact of thepotential risk of mixed infections. The goal of this review is to explore that impact. S.
maltophilia is often isolated from the respiratory tract together with other Gram-negative
species, and yields at least two β-lactamases. The enzymes show the capacity to hydrolyze
a large amount of imipenem and ceftazidime, and exhibit a susceptibility to aztreonam in
combination with cefozopran. The last section elaborates on the idea that S. maltophilia
can assist in the survival of other imipenem-susceptible bacteria such as Serratia marcescens
and Pseudomonas aeruginosa. This theory is also valid for ceftazidime-susceptible P.
aeruginosa. The present review confirms the potential threat of S. maltophilia as an indirect
pathogen, and brings an often ignored fact to light: even originally fragile bacteria canlive through a strong antimicrobial attack in the presence of a helper bacteria such as S.
maltophilia. Although it is sometimes difficult to attribute a causative role to this bacterium,in fact, the existence of S. maltophilia is worthy of attention.
Key words: indirect pathogenicity; β-lactamase; Stenotrophomonas maltophilia

Gram-negative bacilli as a cause of hospitalacquired infection has recently been receiving increasing attention, and its resistance to multidrug
therapy baffles medical workers (Avison et al.,
2001; Livermore, 2002). Hanberger et al. (2001)
reported that immunodeficiency is the most important factor in infections by multidrug-resistantstrains, and in addition, improper usage of broadspectrum antimicrobial agents accompanied withprolonged hospital stays may contribute to thisphenomenon (Denton and Kerr, 1998; Geiger et al.,2001). Among Gram-negative bacilli, multidrugresistant Pseudomonas aeruginosa has been solely

designated as a notifiable organism in Japan since1999. Nonetheless, this step is apparently incomplete because the resistant mechanism differs from
species to species (Kataoka et al., 2001). Cliniciansneed to recognize Stenotrophomonas maltophilia as
an important nosocomial pathogen because thisbacterium retains a noticeable resistance to multidrug treatment and disinfectants. This review
focuses on studies about the clinical aspects of βlactam-resistant S. maltophilia.
The plan of the review is as follows: Section I
outlines the significance of S. maltophilia among
Gram-negative bacilli; Section II considers clinicalAbbreviations: BTB, bromothymol blue; CFU, colony forming unit; MIC, minimum inhibitory concentration; SMA,
sodium mercaptoacetic acid91

D. Kataoka and Y. Tanaka

Gram-negative bacilli
facultative aerobic

aerobic(glucose-nonfermentative)

Family Enterobacteriaceae(= enterobacteria)

Pseudomonas

Escherichia

Stenotrophomonas

Klebsiella

Gram-negative cocci
Serratia
Neisseria
Family Vibrionaceae

Gram-positive cocci
VibrioStaphylococcus
Family Pasteurellaceae
Streptococcus
HaemophilusEnterococcus
anaerobic

Gram-positive bacilli
Bacteroides
Bacillus

Fig. 1. Schema of bacteriological classification. Solid lines, grouping by Gram stain. Dotted lines, grouping by
metabolism. Ellipse, typical genus.

properties including habitat and antimicrobial
resistance. In Section III, we present differential
activities of β-lactamases, such as hydrolytic activity and susceptibility to aztreonam as an inhibitor.
Section IV describes indirect pathogenicity, that is,hidden risks in mixed infections.

I. Significance of S. maltophilia among
Gram-Negative Bacilli
The emergence of β-lactam-resistant Gram-negative bacilli has increasingly been recognized on the
grounds that β-lactam agents came to be used ex92

Clinical aspects of S. maltophilia

Table 1. Number (%) of broad-spectrum β-lactamresistant* strains for bacterial species

tensively in wards (Livermore, 1995). β-Lactam
agents are classified into several groups according
to chemical structure. In general, penicillins arestill effective against Gram-positive cocci, while
they are weak in fighting Gram-negative bacilli.
The newer the cephalosporins and cephamycins,
the wider the antimicrobial spectrum. On one hand,the antimicrobial spectrum of monobactams is limited to Gram-negative bacilli in contrast to penicillins. On the other hand, the spectrum of carbapenems covers almost all Gram-negative bacteria(Kataoka et al., 2002a).
The antimicrobial susceptibilities of Gramnegative bacilli were surveyed at Tottori University
Hospital in 2001. As a matter of course, Gramnegative cocci and Gram-positive bacteria including Staphylococcus spp., Streptococcus spp., Enterococcus spp. and Bacillus spp. are not intended infections (Fig. 1). The survey showed that 176 strains(6.3%) out of 2785 were resistant to all 3 broadspectrum β-lactam agents, that is, imipenem,
ceftazidime and aztreonam. Most of the 176 strains
are not enterobacteria but glucose-nonfermentative
and aerobic bacteria such as P. aeruginosa, Chryseobacterium spp. and S. maltophilia in particular

Number of strains
176 (100.0)
Bacterial species
Stenotrophomonas maltophilia†
96 (54.5)
Pseudomonas aeruginosa†
34 (19.3)
Chryseobacterium indologenes†
10(5.7)
Chryseobacterium meningosepticum†
7(4.0)
Serratia marcescens‡
6(3.4)
Acinetobacter spp.†4(2.3)
Other species
19 (10.8)* Resistant to imipenem, ceftazidime and aztreonam.
† Glucose-nonfermentative bacteria.
‡ Enterobacteria.

(Table 1). These bacteria occupy ecological niches
both inside and outside hospitals, and cause opportunistic infection or iatrogenic transmission. The
growth of these bacteria is slower than that ofenterobacteria; however, glucose-nonfermentative
bacteria are often of resistant to multidrug treatment
and disinfectants (Mori, 2000). Classical resistanceto β-lactam agents in these bacterial species is usually due to an alternation of penicillin-binding
proteins, changes in outer membrane permeability
to β-lactams and most frequently, intrinsic β-lactamase
Table 2. Classification of β-lactamases
production (Gál et al., 2000;
Fang et al., 2002). There is a
Molecular class
Main bacterial species
likelihood of invalidity with
Serine-β-lactamase A (penicillinase)
Haemophilus influenzae
standard chemotherapy beKlebsiella oxytoca
cause of hydrolysis by the βKlebsiella pneumoniae
lactamases.
Proteus mirabilis
Stenotrophomonas maltophilia
The exact opposite of
glucose-nonfermentative
bacC (cephalosporinase) Citrobacter freundii
teria, most enterobacteria areEnterobacter cloacae
Morganella morganii
naturally susceptible to β-lacPseudomonas aeruginosatams (Kataoka et al., 2002a).
Serratia marcescens
However, an overdose of βD (oxacillinase)Pseudomonas aeruginosalactams during hospitalization
causes an emergence of AmpC
Metallo-β-lactamase B (carbapenemase)Aeromonas hydrophila
β-lactamase, whose gene is
Bacillus cereus
Bacteroides fragilis
commonly found on the chroBurkholderia cepacia
mosomes of several enteroStenotrophomonas maltophilia
bacteria species and is typically
From Kuga and Inoue, 2002 with some modifications.
induced by ceftazidime or imi93

D. Kataoka and Y. Tanaka

penem (Coudron et al., 2000; Kataoka
et al., 2002b). Therefore, the treatment
against enterobacteria should be considered carefully. In passing, AmpC isgrouped into class C (cephalosporinase) of Ambler’s prestigious classification (Table 2) (Ambler, 1980).
AmpC β-lactamase of Serratia marcescens is presented in Section IV
once again.

II. Clinical Properties
Although S. maltophilia is a littleFig. 2. Acidity of bacterial colony. Both media contain glucose andknown microbe, it was the 5th Grambromothymol blue (BTB), which is a pH-indicator. On the left plate,colorless colonies (S. maltophilia) are observed because it does not pronegative species according to theduce any acid from glucose. S. maltophilia needed to metabolize pepnumber of isolates found at Tottori
tone for growth, and some alkalis made from peptone have turned the
University Hospital in 2001, after P.
appearance of colonies bluish. On the right plate, yellow colonies (S.
aeruginosa, Escherichia coli, Klebsimarcescens) are grown with a fair amount of acidity by fermentation ofella pneumoniae and S. marcescens
glucose.
(Kataoka et al., 2002a). S. maltophilia
is a Gram-negative aerobic bacillus
S. maltophilia is found in a wide variety of en(Fig. 1), having a strictly respiratory type ofvironments, and has been recovered from nosocometabolism with oxygen as the terminal electron
mial water sources including ice-making machines,
acceptor (Park, 1967; Snell and Lapage, 1971). As
sink traps and nebulizers (Sattler, 2000; Rogues et
Hugh and Ryschenkow (1961) noted, a keyal., 2001). Despite the fact that almost all strains ofdistinguishing feature of S. maltophilia is acidS. maltophilia are isolated from inpatients, Denton
production not from glucose but from maltose (Fig.
et al. (1998) maintain that person-to-person trans2 and Table 3). The term “maltophilia” seems tomission is an infrequent occurrence in the nosococome from the bacterium’s preference for maltose.
mial setting. Epidemiological research of S. maltoIt goes without saying that all enterobacteria
philia also highlights that babies and elderly people
including S. marcescens ferment glucose, andare more likely to get this bacterium than those inproduce some acid and/or gas (Sakazaki, 1992).
the generations in between. As to the sources, they
are also usually isolated from the respiratory tractsuch as sputum and the broncho-alveolar lavage.
Table 3. Acid production from sugars
The respiratory tract is the most common siteof S. maltophilia’s isolation in hospitalized paBacterial species
tients;however, most patients are colonized rather
Sugar S. maltophilia S. marcescens P. aeruginosathan infected at this site (Geiger et al., 2001). As
Glucose–F
O
Rogues et al. (2001) have stated: A ventilator can be
Lactose–––colonized with the patient’s endogenous flora andMaltoseOF–
becomes a hotbed of secondary infections. In ourF, fermentation; O, oxidation; –, negative of acid proinvestigation, more than 10 strains of S. maltophilia
duction. From Kataoka et al., 2003b with some modifications.
were actually separated from superinfections of94

Clinical aspects of S. maltophilia

pneumonia, bacteremia or cholangitis (Kataoka etal., 2003b). S. maltophilia has been associated withan expansive spectrum of clinical syndromes:
endocarditis, conjunctivitis, keratitis, urethritis,
ulcerative colitis and Crohn’s disease (Denton andKerr, 1998).
S. maltophilia has been reported to be resistant
to extended-spectrum cephalosporins, carbapenems
and aminoglycosides (Denton et al., 1999; BarbierFrebourg et al., 2000). The medical community has
increasingly become dependent on carbapenems,
because of their high affinity for penicillin-binding
proteins, excellent penetration across bacterial outer membranes, and resistance to hydrolysis by the
majority of serine-based β-lactamases (Payne et al.,
1997; Rasmussen and Bush, 1997; Yin et al., 2003).
Therefore, carbapenems are so-called the final
trumps. But all strains really revealed resistance toimipenem grouped into carbapenems (Table 4), andthis peculiarity coincides with former documents(Hanberger et al., 2001; Simm et al., 2002). Ceftazidime appeared to possess reasonable activity
against some strains (Table 4), but as resistance ishighly variable, ceftazidime cannot be used inempiric chemotherapy (Geiger et al., 2001). βLactamase inhibitor such as sulbactam was exceptionally effective, so inhibition of β-lactamases
could be a key to the problem. This hypothesis
leads to the Discussion in the next Section.

This Section argues that S. maltophilia canlive through the selective pressure of standard
chemotherapy using β-lactams, and get its chancein the limelight. For this reason, clinicians should
attach more importance on information from theclinical laboratory in order not to overlook this socalled “reserved” organism.

III. Differential Activities of β-Lactamases
The most common theory is that S. maltophilia
chromosomally yields at least 2 inducible β-lactamases, a class B metallo-β-lactamase (carbapenemase) L1 and a class A active-site serine-β-lactamase (penicillinase) L2 (Table 2) (Ambler, 1980).
Both enzymes mediate an essential resistance to βlactam agents as a result of cutting the carbonnitrogen bond of β-lactam rings off (Ullah et al.,
1998; Avison et al., 2001; Spencer et al., 2001). There
are still no clinically useful inhibitors of metallo-βlactamases in contrast with serine-β-lactamases(Simm et al., 2002). The expression of L1 and L2
has been shown by coinducibility under the existence of a single β-lactam agent as an inducer, andalso by some single site mutations resulting in overexpression of both genes (Avison et al., 2002).
In recent years, the sodium mercaptoacetic
acid (SMA) disk was developed for the screening of
metallo-β-lactamase-producing Gram-negative
bacilli (Arakawa et al., 2000). Though the main
target of this thiol compound is originally IMP-1producing P. aeruginosa or S. marcescens, this convenient test can be applied to L1-producing S. maltophilia. Even the imipenem-resistant P. aeruginosa strain indicated no reaction, whereas S. maltophilia showed good susceptibility to SMA (Fig. 3).
The resistance to imipenem of that P. aeruginosastrain is not due to production of carbapenemase,but to mutational impermeability of up-regulated
MexEF-OprN and reduced OprD (Livermore,2002).
The hydrolytic activities of L1 and L2 β-lactamases have already been measured (Kataoka etal., 2003a). Decrease in UV absorption at an appro-

Table 4. Antimicrobial susceptibility (%) of S.
maltophilia
Antimicrobial
S. maltophilia [n = 155]
agent
Susceptible* Intermediate* Resistant*
Piperacillin†
11 (7.1)
Cefsulodin†
3 (1.9)Ceftazidime
35 (22.6)
Sulbactam/
Cefoperazone 105 (67.7)
Aztreonam
4 (2.6)
Imipenem
0 (0.0)0 (0.0)9 (5.8)24 (15.5)

143 (92.9)
142 (92.2)
96 (61.9)

41 (26.5)
0 (0.0)0 (0.0)9 (5.8)151 (97.4)
155 (100.0)* Categories are recommended by the National Committee for Clinical Laboratory Standards (2000).
† No data with 1 isolate.

95

D. Kataoka and Y. Tanaka(Lister et al., 1998; Song et al., 2003).
These reports are in harmony with theproposal of Nishida et al. (1999) that amonobactam derivative can indicate
considerable synergy with cefpirome.
Upon further examination, no synergyof aztreonam with imipenem isexhibited. These results also explainthat aztreonam tends not to inhibit
carbapenemases, because imipenem
should be broken down only bycarbapenemase L1. Therefore, thesynergy of aztreonam with cefpirome
was probably due to the aztreonam’s
inhibition of penicillinase L2.
Fig. 3. Inhibition by sodium mercaptoacetic acid (SMA). On the left
plate (S. maltophilia), a considerable expansion in zone size around an
The inhibitory activity of aztreimipenem (IPM) disk close to SMA is revealed, in contrast to IPM
onam against the penicillinases of S.
without SMA. That means positive of carbapenemase. On the rightmaltophilia has been explored (Kataoka
plate (P. aeruginosa, which is yellow-green due to the fluorescent pigand Tanaka, in press). All 5 strains ofment), no expansions between the two IPM disks are shown. That
S. maltophilia had already been connaturally means negative of carbapenemase despite resistance to IPM.
firmed susceptible to clavulanic acid
From Kataoka et al., 2003b.
so they were identified possessors of
L2 β-lactamase (Fang et al., 2002;
Coudron et al., 2003). In spite of little synergy ofpriate wavelength means hydrolysis of the β-lactam
imipenem to all 5 strains, a considerable synergy ofring (Samuni, 1975). By inactivation for 4 h, an avaztreonam with cefozopran was found in 4 strains
erage reduction of approximately 20% was shownout of 5 (Table 5). Sulbactam is definitely known as
in both absorbances of imipenem and ceftazidime
a potent inhibitor of penicillinases (Mahgoub and(Fig. 4). Taking into consideration the affinity, theAly, 1998), and actually showed an excellent
hydrolysis of imipenem is performed by L1, andthat of ceftazidime is largely related to L2. It appears that the hydrolytic function of L2 is equivalent to that of an extended-spectrum β-lactamase.
The notion of coordinated expression allowed
Krueger et al. (2001) to point out that the bacterial
primary resistance to β-lactams is not overcome
until both β-lactamases are inhibited. Although βlactam agents are thought to be unrealistic tools
against infections with S. maltophilia, this bacterium is useful as an extreme model of a β-lactamase
producer. Our inhibition test notes that imipenem
and cefpirome are predominantly hydrolyzed by
L1, whereas ceftazidime is predominantly hydrolyzed by L2 (Kataoka et al., 2003a). Incidentally,aztreonam grouped into monobactams has beenreported as an inhibitor of extracellular β-lactamase(%)

Absorbance

100

90

Imipenem

80

Ceftazidime

70

0

1

2

3

4 (h)

Inactivation time

Fig. 4. Hydrolysis of β-lactam agents by S. maltophilia.
The absorbances of the β-lactam rings were measured by
UV spectrophotometry. Closed circle, imipenem; closed
square, ceftazidime.

96

Clinical aspects of S. maltophilia

Table 5. MICs against S. maltophilia
MIC (µg/mL)
Aztreonam Imipenem Aztreonam/ Cefozopran Aztreonam/
Imipenem
Cefozopran

StrainS. maltophilia TOT8
S. maltophilia TOT16
S. maltophilia TOT19
S. maltophilia TOT43
S. maltophilia TOT57

256
1024
1024
256
512

512
512
256
256
256

512/512512/512256/256
256/256
256/256

256
256
256
128
128

256/256
128/128
128/128
64/64
64/64

MIC, minimum inhibitory concentration. From Kataoka and Tanaka, in press withsome modifications.

synergy with cefoperazone (not cefozopran) against
S. maltophilia (Table 4). These findings support theidea that aztreonam as well as sulbactam inhibits
penicillinases instead of performing aztreonam’s
own antimicrobial activity. This account isdefended by both the high resistant frequency of
aztreonam and the high MICs of aztreonam alone
against S. maltophilia (Tables 4 and 5). Figure 5

presents the antimicrobial effect of the combinationof aztreonam and cefozopran. The colony forming
unit (CFU) counts at 6 h in cultures treated withaztreonam-cefozopran were approximately 1 log
lower than those in cultures treated with cefozopranalone, and 2 logs lower than those treated withaztreonam alone.
Aztreonam is additionally suitable in combination chemotherapy for Gram-negative infections.
One reason is that monobactams have no antimicrobial activity against Gram-positive bacteria: The
risk of superinfection by Gram-positive bacteria,
for instance enteritis caused by Staphylococcus
aureus, is estimated to be low. Improper usage ofbroad-spectrum antimicrobial agents might prepare
a hotbed of multidrug-resistant strains; therefore,
the minimum usage of narrow-spectrum ones has
recently been recommended for the control of
hospital-acquired infections. We hope that a reduction in the dosage of cefozopran by employing
aztreonam comes to not only obtain antimicrobialsynergy and minimize toxicity but also to preventthe emergence of penicillinase producers.

(CFU/mL)

Antimicrobial
agent-free

7

6

Log10

Aztreonam

Cefozopran5

Aztreonam/Cefozopran
4

0

2

4

6 (h)

IV. Indirect Pathogenicity

Incubation time

Fig. 5. Colony forming unit (CFU) count of S. maltophilia in culture treated with antimicrobial agents as follows: Cross, antimicrobial-agent free; closed circle,
aztreonam; open square, cefozopran; closed square,
aztreonam and cefozopran. From Kataoka and Tanaka,in press with some modifications.

A popular idea exists nowadays that S. maltophilia
is of strictly limited pathogenicity (Denton and Kerr,
1998), while this organism has the capacity to hydrolyze a variety of β-lactam antimicrobial agentsand the possibility of assisting the survival of other97

D. Kataoka and Y. Tanaka(A) Without S. maltophilia
β-lactamβββbactericidal
actionβsusceptible bacteria(B) With S. maltophilia
ilia

β

L1

toph

al
S. m

bind
L1

L2

L2
β-lactamaseβhydrolysis of
β-lactams

growth ofsusceptible
bacteria

β

β
β

β

active β-lactams

βhydrolyzed (inactivated) β-lactams

β-lactamase

Fig. 6. Schema of indirect pathogenicity.
A: Without S. maltophilia, susceptible bacteria are killed by β-lactam agents.
B: With S. maltophilia, β-lactamases leaking from S. maltophilia hydrolyze β-lactam agents, so even susceptible
bacteria can survive.

susceptible pathogens. S. maltophilia is apparently
a β-lactamase supplier rather than a direct cause of
disease where it is confronted with β-lactam agents.
It is generally isolated from severely immunosuppressed hosts because of its weak virulence, andhas frequently been isolated together with otherGram-negative species in our observations (Kataoka
et al., 2003b). It looks like broad-spectrum antimicrobial therapy takes aim at the major opportunistic pathogens such as P. aeruginosa and S. mar-cescens in most cases of Gram-negative infection.
As it turns out, the existence of S. maltophilia is apt
to be less reflected in the choice of antimicrobial
agents. However, broad-spectrum β-lactam agents
could act as a sieve of multidrug-resistant strains
such as S. maltophilia and serve to induce theirown β-lactamases (Kataoka et al., 2001, 2002b).
That is why we propose the concept of indirect
pathogenicity (Fig. 6): A chain of the preceding
processes can result in the encouragement of mixed98

Clinical aspects of S. maltophilia

Log10

ceftazidime (Raimondi et al., 2001).
We consider that both β-lactamases of
L2 and AmpC were yielded duringexposure to ceftazidime in mixed
culture, and have assumed that L2
leaking from S. maltophilia hid itself
behind S. marcescens’ own AmpC. As
proof of the output of L2, P. aeruginosa confronted with a large quantityof ceftazidime could grow under the
support of S. maltophilia (Kataoka etal., 2003b). As Livermore (2002) has
demonstrated, P. aeruginosa also harbors AmpC β-lactamase, but unlike S.
marcescens the level of resistance deFig. 7. Mixed culture of S. marcescens and S. maltophilia. On the top
pends on the degree of depression.
left plate, an extremely large number of yellow colonies (S.
Following this thesis, the P. aeruginomarcescens) are seen (none for imipenem was supplemented). On thesa strain does not seem to have protop right plate, the number of yellow colonies is obviously decreased
because of 16 µg/mL of imipenem in the medium. On the bottom plate,
duced enough β-lactamases to protect
however, a fairly large number of yellow colonies and colorless coloitself from ceftazidime.
nies (S. maltophilia) are indicated even under exposure to 16 µg/mL
Though a multidrug-resistantimipenem (from Kataoka et al., 2003b).
strain is not always relevant to indirect
pathogenicity, at least our positive
findings can be thought of as a formal
infections caused by another pathogen, even if S.
explanation for some potential threat: Even an
maltophilia originally colonizes in the patient’s resordinary bacterium can endure antimicrobial attack
piratory tract.
The indirect pathogenicity of S. maltophilia
has already been confirmed (Kataoka et al., 2003b).
(CFU/mL)
The CFU of the yellow colonies (S. marcescens and
6****
P. aeruginosa) was counted for each plate, because
both S. marcescens and P. aeruginosa form yellow
5colonies by acid production from glucose incontrast with colorless colonies of S. maltophilia(Figs. 2 and 7).
4
Figure 8 demonstrates that S. marcescens exposed to imipenem increased in the existence of S.
maltophilia. This concept is also applicable to a3
mixed culture of P. aeruginosa with S. maltophilia(µg/mL)
4
16(Kataoka et al., 2003b). On the other hand, the CFU
Concentration of imipenem added in culture
count of S. marcescens seemed not to be influenced
Fig. 8. Colony forming unit (CFU) count of S. marcesin cases when it was faced to ceftazidime instead ofcens in culture which contains imipenem. Pale bar, pure
imipenem (Kataoka et al., 2003b), because S.
culture of S. marcescens; dark bar, mixed culture of S.
marcescens is a potential owner of AmpC βmarcescens with S. maltophilia. **P < 0.01 with t-test.
lactamase (Coudron et al., 2000). An induction of
The bars represent means plus SD. From Kataoka et al.
AmpC makes the strain highly resistant to(2003b) with some modifications.
99

D. Kataoka and Y. Tanaka

by the assistance of β-lactamases from an indirect
pathogen, namely S. maltophilia.

negative bacteria by using thiol compounds. J Clin
Microbiol 2000;38:40–43.
3 Avison MB, Higgins CS, von Heldreich CJ, Bennett
PM, Walsh TR. Plasmid location and molecular
heterogeneity of the L1 and L2 β-lactamase genes of
Stenotrophomonas maltophilia. Antimicrob Agents
Chemother 2001;45:413–419.
4 Avison MB, Higgins CS, Ford PJ, von Heldreich CJ,
Walsh TR, Bennett PM. Differential regulation of
L1 and L2 β-lactamase expression in Stenotrophomonas maltophilia. J Antimicrob Chemother 2002;
49:387–389.
5 Barbier-Frebourg N, Boutiba-Boubaker I, Nouvellon
M, Lemeland JF. Molecular investigation of Stenotrophomonas maltophilia isolates exhibiting rapid
emergence of ticarcillin-clavulanate resistance. J
Hosp Infect 2000;45:35–41.
6 Coudron PE, Moland ES, Thomson KS. Occurrence
and detection of AmpC beta-lactamases among
Escherichia coli, Klebsiella pneumoniae, and Proteus mirabilis isolates at a Veterans Medical Center.
J Clin Microbiol 2000;38:1791–1796.
7 Coudron PE, Hanson ND, Climo MW. Occurrenceof extended-spectrum and AmpC beta-lactamases inbloodstream isolates of Klebsiella pneumoniae:
isolates harbor plasmid-mediated FOX-5 and ACT-1
AmpC beta-lactamases. J Clin Microbiol 2003;41:
772–777.
8 Denton M, Kerr KG. Microbiological and clinicalaspects of infection associated with Stenotrophomonas maltophilia. Clin Microbiol Rev 1998;11:57–
80.
9 Denton M, Todd NJ, Kerr KG, Hawkey PM,
Littlewood JM. Molecular epidemiology of Stenotrophomonas maltophilia isolated from clinical specimensfrom patients with cystic fibrosis and associated environmental samples. J Clin Microbiol 1998;36:1953–
1958.
10 Denton M, Keer V, Hawkey PM. Correlation
between genotype and β-lactamases of clinical andenvironmental strains of Stenotrophomonas
maltophilia. J Antimicrob Chemother 1999;43:555–
558.
11 Fang H, Edlund C, Hedberg M, Nord CE. New
findings in beta-lactam and metronidazole resistantBacteroides fragilis group. Int J Antimicrob Agents
2002;19:361–370.
12 Gál Z, Szabó D, Kovács P, Hernádi F, TóthMartinez B, Rozgonyi F. β-Lactam susceptibility
patterns and investigation of cephalosporin hydrolysing β-lactamases of Gram-negative extraintestinal clinical isolates. Int J Antimicrob Agents 2000;
16:395-400.

Conclusion
Carbapenems and cephalosporins are among themost frequently administered antimicrobial agents
due to their excellent antimicrobial activity and low
toxicity; therefore, these compounds are oftenprescribed for severe Gram-negative infections(Coudron et al., 2003; Yin et al., 2003). The present
study upholds the theory that overuse of these antimicrobial trumps has a profound effect on a selection pressure on account of the induction of βlactamases, and results in the appearance of an
antibiotic-free area around S. maltophilia. In otherwords, sulbactam, minocycline and trimethoprimsulfamethoxazole, relatively unpopular but effective drugs, should be selected more in a case wheremixed infection occurs (Kataoka et al., 2002a).
Although it is sometimes difficult to attribute a
causative role to S. maltophilia when the organism
is isolated from respiratory secretions alone(Sattler, 2000), throughout the course of this essay
we establish that S. maltophilia is worthy of attention. Thus, a full understanding of the indirect pathogenicity in vivo awaits future study.

Acknowledgments: We are grateful to Mr. Hiromitsu
Fujiwara and Ms. Ayako Tanimoto, Division of ClinicalLaboratory, Tottori University Hospital, for their technical assistance. We also appreciate the advice and expertise of Professor Shiro Ikawa, Division of Clinical Laboratory Medicine, Department of Pathophysiological andTherapeutic Science, School of Medicine, Tottori University Faculty of Medicine.

References
1 Ambler RP. The structure of β-lactamases. Phil
Trans R Soc Lond B 1980;289:321–331.
2 Arakawa Y, Shibata N, Shibayama K, Kurokawa H,
Yagi T, Fujiwara H, et al. Convenient test forscreening metallo-β-lactamase-producing gram-100

Clinical aspects of S. maltophilia

13 Geiger AM, Hogardt M, Heesemann J. Burkholderia/Stenotrophomonas. Contrib Microbiol 2001;
8:20–34.
14 Hanberger H, Diekema D, Fluit A, Jones R,
Struelens M, Spencer R, et al. Surveillance of antibiotic resistance in European ICUs. J Hosp Infect2001;48:161–176.
15 Hugh R, Ryschenkow E. Pseudomonas maltophilia,an Alcaligenes-like species. J Gen Microbiol 1961;
26:123-132.
16 Kataoka D, Kawakami T, Fujiwara H, Tanaka Y,
Tanimoto A, Ikawa S, et al. The antibiotic resistanceof gram-negative bacilli isolated in Tottori University Hospital. Tottori Igaku Zasshi 2001;29:161–
167 (in Japanese).
17 Kataoka D, Fujiwara H, Tanimoto A, Tanaka Y. The
results of antibiotic susceptibility of gram-negativerods at Tottori University Hospital: the possibility ofthe sensible use of narrow-spectrum antimicrobial
agents. Kansenshogaku Zasshi 2002a;76:542–549(in Japanese with English abstract).
18 Kataoka D, Fujiwara H, Tanimoto A, Tanaka Y.
Relation between hospitalization and the resistanceof enterobacteria to β-lactam antimicrobial agents.
Kansenshogaku Zasshi 2002b;76:1045–1047.
19 Kataoka D, Fujiwara H, Tanimoto A, Ikawa S,
Tanaka Y. The differential β-lactamase activity of
Stenotrophomonas maltophilia. J Hosp Infect2003a;54:246–248.
20 Kataoka D, Fujiwara H, Kawakami T, Tanaka Y,
Tanimoto A, Ikawa S, et al. The indirect pathogenicityof Stenotrophomonas maltophilia. Int J Antimicrob
Agents 2003b;22:601–606.
21 Kataoka D, Tanaka Y. The combination of aztreonamand cefozopran against Stenotrophomonas maltophilia. J Infect Chemother (in press).
22 Krueger TS, Clark EA, Nix DE. In vitro susceptibility of Stenotrophomonas maltophilia to various
antimicrobial combinations. Diagn Microbiol Infect
Dis 2001;41:71–78.
23 Kuga A, Inoue M. Antimicrobial capacity. Rinsho
To Biseibutsu 2002;29:3–11 (in Japanese).
24 Lister PD, Sanders WE, Sanders CC. Cefepimeaztreonam: a unique double β-lactam combination
for Pseudomonas aeruginosa. Antimicrob Agents
Chemother 1998;42:1610–1619.
25 Livermore DM. β-Lactamases in laboratory andclinical resistance. Clin Microbiol Rev 1995;8:557–
584.
26 Livermore DM. Multiple mechanisms of antimicrobial resistance in Pseudomonas aeruginosa: ourworst nightmare? Clin Infect Dis 2002;34:634–640.
27 Mahgoub H, Aly FA. UV-spectrophotometric deter-

mination of ampicillin sodium and sulbactam sodium in two-component mixtures. J Pharm Biomed
Anal 1998;17:1273–1278.
28 Mori T. Identification of Gram-negative bacilli. In:
Oguri T, ed. Clinical microbiology handbook.
Tokyo: Miwa Syoten; 2000. p. 82–96 (in Japanese).
29 National Committee for Clinical Laboratory Standards. Methods for dilution antimicrobial susceptibility tests for bacteria that grow aerobically. In:
Approved standard, 5th ed. NCCLS document M7A5. Wayne (PA): National Committee for ClinicalLaboratory Standards; 2000.
30 Nishida K, Kunugita C, Uji T, Higashitani F, Hyodo
A, Unemi N, et al. In vitro and in vivo activities ofSyn2190, a novel β-lactamase inhibitor. Antimicrob
Agents Chemother 1999;43:1895–1900.
31 Park RWA. A comparison of two methods for detecting attack on glucose by Pseudomonads andAcromobacters. J Gen Microbiol 1967;46:355–360.
32 Payne DJ, Bateson JH, Gasson BC, Proctor D,
Khushi T, Farmer TH, et al. Inhibition of metallo-βlactamases by a series of mercaptoacetic acid thiol
ester derivatives. Antimicrob Agents Chemother
1997;41:135–140.
33 Raimondi A, Sisto F, Nikaido H. Mutation in
Serratia marcescens AmpC β-lactamase producing
high-level resistance to ceftazidime and cefpirome.
Antimicrob Agents Chemother 2001;45:2331–2339.
34 Rasmussen BA, Bush K. Carbapenem-hydrolyzing
β-lactamases. Antimicrob Agents Chemother
1997;41:223–232.
35 Rogues AM, Maugein J, Allery A, Fleureau C,
Boulestreau H, Surcin S, et al. Electronic ventilator
temperature sensors as a potential source of respiratory tract colonization with Stenotrophomonas maltophilia. J Hosp Infect 2001;49:289–292.
36 Sakazaki R. Definition of the family Enterobacteriaceae. In: Sakazaki R, Tamura T, eds. Enterobacteria.
Tokyo: Kindai Syuppan; 1992. p. 6–17 (in Japanese).
37 Samuni A. A direct spectrophotometric assay anddetermination of Michaelis constants for the βlactamase reaction. Anal Biochem 1975;63:17–26.
38 Sattler CA. Stenotrophomonas maltophilia infection
in children. Pediatr Infect Dis J 2000;19:877–878.
39 Simm AM, Higgins CS, Carenbauer AL, Crowder
MW, Bateson JH, Bennett PM, et al. Characterization of monomeric L1 metallo-β-lactamase and therole of the N-terminal extension in negative cooperativity and antibiotic hydrolysis. J Biol Chem 2002;
277:24744–24752.
40 Snell JJS, Lapage SP. Comparison of four methods
for demonstrating glucose breakdown by bacteria. J
Gen Microbiol 1971;68:221–225.

101

D. Kataoka and Y. Tanaka

philia at 1.7 Å resolution. J Mol Biol 1998;284:125–
136.
44 Yin XH, Mikamo H, Tamaya T. Nosocomial infectious potency of imipenem-resistant Pseudomonas
aeruginosa isolated from obstetric and gynecologic
infections. J Infect Chemother 2003;9:97–100.

41 Song W, Woo HJ, Kim JS, Lee KM. In vitro activityof β-lactams in combination with other antimicrobial
agents against resistant strains of Pseudomonas
aeruginosa. Int J Antimicrob Agents 2003;21:8–12.
42 Spencer J, Clarke AR, Walsh TR. Novel mechanismof hydrolysis of therapeutic β-lactams by Stenotrophomonas maltophilia L1 metallo-β-lactamase. J
Biol Chem 2001;276:33638–33644.
43 Ullah JH, Walsh TR, Taylor IA, Emery DC, Verma
CS, Gamblin SJ et al. The crystal structure of the L1
metallo-β-lactamase from Stenotrophomonas malto-

Received September 9, 2003;accepted October 23, 2003
Corresponding author: Yoshinori Tanaka, MD, PhD

102