sou · GLB3132d3 · 1998-01-01 00:00:00 — Avilamycin is a mixture of oligosaccharides of the orthosomycin group, that are produced by 3TREPTOMYCES VIRIDOCHROMOGENES. Other members of this group include curamycin and everninomycins (, 1973). Avilamycin is mainly against gram-positive bacteria. The compound is used for growth promotion in swine and poultry, at dosages ranging from 5 to 40 ppm for swine and from 2.5 to 10 ppm for poultry. It has never been used for therapeutic purposes in either human or veterinary medicine but a related compund, everninomycins, has been suggested for evaluation in human therapy (Chopra ET AL , 1997; Nicas ET AL , 1997). In rats and swine, oral avilamycin is primarily excreted in faeces (, 1991). A.2 Mode of action and resistance mechanisms Avilamycin acts on the bacterial ribosome, by inhibiting the binding of formylmethionyl-tRNA to the 30 S ribosomal subunit (, 1973). This blocks the formation of the 70 S initiation complex in bacterial protein synthesis. This inhibition occurs in IN ribosomal systems from both gram-positive and gram-negative bacteria (, 1973), so the difference in avilamycin susceptibility of gram-positive and gram-negative bacteria is probably due to differences in factors outside the protein synthesizing system, like cell wall composition. Experimental data indicate that avilamycin interferes with the attachment of tRNA to the ribosome by binding to the 30 S subunit (, 1973). No data has been published on mechanisms of resistance, but it seems logical to assume that structural changes in the ribosomal 30 S subunit or ribosomal protection could confer resistance to avilamycin in naturally susceptible bacteria. In view of the ease and speed at which resistance to some other anti-ribosomal antimicrobials (e.g. aminoglycosides and tetracyclines) has emerged, it is very important to investigate possible resistance mechanisms to avilamycin and how these may interfere with the effect of other anti-ribosomal substances. Full cross-resistance to everninomycins would be expected in avilamycin- resistant bacteria, as the two compounds are structurally very similar (see figure A.I). Cross-resistance to unrelated antimicrobials in avilamycin- resistant bacteria has not been reported, but there is no information on The figures in this report is only available in the printed version Tentative sketch of avilamycin and everninomycins possible cross-resistance to antimicrobials with a similar site of action (i.e. the binding site for formylmethionyl tRNA), such as viomycin and capreomycin, among others. Cross-resistance between structurally unrelated compounds may be seen when these compounds have the same site of action. Even when the sites of action are not identical, cross-resistance could, theoretically, appear if structural changes in the ribosome, or protecting proteins, affect both antimicrobial binding sites. In view of this, knowledge of resistance mechanisms is necessary in order to asses possible risks regarding resistance. A.3 Development of resistance Unfortunately, only two publications concerning bacterial susceptibility to avilamycin have been found. This is enough to illustrate the prevalence of avilamycin resistance, but the data from these studies are compiled in table A.I. Reported prevalence of avilamycin resistance Devriese ET AL, In , of the annual consumption of avilamycin in feed is used in poultry, which is reflected in the differences in resistance to this compound in % FAECIUM from cattle swine and poultry, respectively. consumption of avilamycin increased four-fold between 1994 and 1995, coinciding with its introduction into poultry feed. Unfortunately, there is no data available on avilamycin resistance in %NTEROCOCCUS FAECIUM isolates collected before this increase in the consumption. As no data has been published on the development of resistance, it is not possible to evaluate the risk of this. An adequate amount of this type of data is essential in risk evaluation and should be made available as soon as possible. A.4 Acquisition of resistance No published information has been found about genes conveying resistance to avilamycin, transfer of avilamycin resistance, or bacterial hosts for resistance genes. If no such investigations have been undertaken, they should be planned immediately. A.5 Impact of resistance on animal and human health As avilamycin is not yet used for therapy in humans or animals, resistance would not be expected to cause clinical problems unless cross-resistance to other substances was present. However, in the future orthosomycin compounds may well be of interest for clinical use. For example, everninomycins have been suggested as a new important candidate for use in human therapy (Cormican and , 1996; , 1996; Chopra ET AL , 1997; Nicas ET AL , 1997). The impact of avilamycin resistance on the life span of future drugs is difficult to assess, but could be substantial. A.6 Other effects on the microflora Hinton (1988) investigated the effect of in-feed avilamycin on salmonella colonisation in chickens. Groups of 10 chickens were given avilamycin in the feed at concentrations of 2.5 or 10 ppm, alone or together with monensin. Medicated birds were compared to non-medicated controls. Four replicate experiments were performed, in two study designs, where the birds were either initially infected on the day of purchase or one week later. were infected with 3ALMONELLA Kedougou in the feed for two weeks, at concentrations from 1.6 to 176 bacterial cells per g feed. All birds in the same replicate experiment received the same dose of organisms. Samples for bacterial culture were taken on day 7 and 14 after the introduction of infected feed. The author concluded that no evidence was obtained to suggest that avilamycin, at concentrations of 2.5 or 10 ppm in the feed, favoured colonisation of the intestinal tract in chickens with 3 Kedougou when they were challenged with this organism in the feed. This is the only published study on the effect of avilamycin on intestinal salmonellae. One single study, no matter how well performed, is enough to form the basis for any definite conclusions, especially when this study did not result in any clear evidence as to whether avilamycin presents a risk in this aspect or not. A.6.2 Other enteric pathogens No publications on the effects of avilamycin on other enteric phogens have been found. A.7 Effects on specific animal diseases Avilamycin at growth promoting levels has been shown to reduce the amount of #LOSTRIDIUM PERFRINGENS in the intestinal tract of chickens (Elwinger ET AL , 1993; Elwinger ET AL , 1995) and may thus be used prophylactically against necrotic enteritis in poultry. (1989) investigated the effect of avilamycin at 40 or 80 ppm in the control of stress-induced post-weaning diarrhoea in piglets. The author stated that avilamycin at 80 ppm had a significant (p<0.05) effect in reducing diarrhoea and mortality in newly weaned piglets. Even at the level of 40 ppm, which may be used for growth promoting purposes in piglets, avilamycin notably reduced diarrhoea, although these figures were not statistically significant. Mortality was, however, significantly (p<0.05) reduced in the piglets that received 40 ppm avilamycin in the feed. At 40 ppm, avilamycin did not have a growth promoting effect in this experiment. Thus, in this study design, with piglets that were already sick, avilamycin served as a therapeutic or prophylactic agent and not as a growth promoter. No information about possible toxic effects either on the target species or on humans has been found. Such effects may be totally absent or, as it is a comparatively new compound, reports on allergy and other side effects may not yet have appeared. One publication (, 1991) has been found concerning residues. The results in this study indicate that avilamycin fed to swine at a concentration of 60 ppm gives rise to small but measurable residues in tissues. At zero withdrawal time the residue levels were 0.14 ppm in muscle, 0.66 ppm in liver, 0.34 ppm in kidney and 0.55 ppm in fat. There is no available information to suggest that this would present a risk for the consumer. Source separated municipal solid waste and agricultural waste can be utilised for biogas production. Substances with an antimicrobial effect against anaerobic bacteria could disturb this process. In studies on manure from pigs and poultry fed avilamycin, (1989) reported efficient operation of experimental and large mesophilic digesters. The presence of avilamycin appeared to alter the metabolism of the microflora, increasing the efficiency to degrade volatile solids. No other publications on possible environmental effects of avilamycin have been found. As the compound is produced by a soil microbe, it would be expected to be microbially degraded in soil. Use of avilamycin in poultry appears to have caused an increase in avilamycin resistance in % FAECIUM from this animal species. might have appeared after only a few years of avilamycin use. Avilamycin is closely related to everninomycins, and cross-resistance could shorten the therapeutic life span of everninomycins if they were to be used in human therapy. The overall information about the possible effects of avilamycin in various aspects is much too scarce to form the basis of a risk assessment. #HOPRA ) (ODGSON * -ETCALF " AND 0OSTE ' , 1997. The search for antimicrobial agents effective against bacteria resistant to multiple antibiotics. !NTIMICROBIAL !GENTS AND #HEMOTHERAPY 497-503. #ORMICAN M ' AND *ONES 2 . , 1996. Emerging resistance to antimicrobial agents in Enterococci, staphylococci and nonpneumococcal streptococci. $!.-!0, 1997. Consumption of antimicrobial agents and occurrence of antimicrobial resistance in bacteria from food animals, food and humans in . $AUBE ' (OMMEZ * AND (AESEBROUCK & , 1993. In vitro susceptibility of #LOSTRIDIUM PERFRINGENS isolated from farm animals to growth- enhancing antibiotics. *OURNAL OF !PPLIED "ACTERIOLOGY 55-57. %LWINGER + %NGSTR¶M " "ERNDTSON % &OSSUM / AND 4EGL¶F " , 1993. Effect ov Avotan (avoparcin) and Maxus (avilamycin) on the caecal growth of #LOSTRIDIUM PERFRINGENS and the occurrence of necrotic enteritis in broilers (report). %LWINGER + %NGSTR¶M " "ERNDTSON % &OSSUM / AND 7ALDENSTEDT , , 1995. Effect of Maxus (avilamycin), Avotan (avoparcin), Monteban (narasin) and Elancoban (monensin-Na) on the caecal growth of #LOSTRIDIUM PERFRINGENS in broilers (report). (INTON - , 1988. Salmonella colonization in young chickens given feed supplemented with the growth promoting antibiotic avilamycin. *OURNAL OF 6ETERINARY 0HARMACOLOGY AND 4HERAPEUTICS 269-275. +YRIAKIS 3 # , 1989. The effects of avilamycin in the control of stress-induced post- weaning diarrhoea in piglets. *OURNAL OF 6ETERINARY 0HARMACOLOGY AND 4HERAPEUTICS 296-301. -AGNUSSEN * $ $ALIDOWICZ * % 4HOMSON 4 $ AND $ONOHO ! , , 1991. Tissue residues and metabolism of avilamycin in swine and rats. *OURNAL OF !GRICULTURAL AND &OOD #HEMISTRY 306-310. .ICAS 4 ) :- , AND "$ + , 1997. Beyond vancomycin: new therapies to meet the challenge of glycopeptide resistance. 4RENDS IN -ICROBIOLOGY 240-249. , .YE * # 0ATTERSON * ! +ELLY $ 4 AND &URUMOTO %LKIN % * , 1989. Effects of avilamycin in swine and poultry wastes on methane production in anaerobic digesters. "IOLOGICAL 7ASTES 35-45. 5RBAN # -ARIANO . -OSINKA 3NIPAS + 7ADEE # #HAHROUR 4 AND 2AHAL * * , 1996. Comparative in-vitro activity of SCH 27899, a novel everninomicin, and vancomycin. *OURNAL OF !NTIMICROBIAL #HEMOTHERAPY 361-364. 7OLF (, 1973. Avilamycin, an inhibitor of the 30 S ribosomal subunits function. &%"3 ,ETTERS 181-6. Bacitracin is a complex mixture of cyclic polypeptides produced by "ACILLUS SUBTILIS and "ACILLUS LICHENIFORMIS. The compound has bactericidal effect on gram-positive bacteria but activity against gram-negative organisms (Prescott and Baggot, 1993). It is most commonly used in complex with zinc which seems to stabilise the antibiotic complex (Quinlan and Gutteridge, 1989). Bacitracin is poorly absorbed from the gastrointestinal tract (Donoso ET AL , 1970; Froyshov ET AL , 1986), as well as from skin and mucosal surfaces. Absorbed bacitracin is excreted by glomerular filtration (Prescott and Baggot, 1993). The substance is used in human therapy, mostly for topical treatment of superficial infections of the skin and mucosal surfaces. However, its effectiveness against vancomycin-resistant enterococci has led to an increase in its use for oral treatment (ODonovan ET AL , 1994; , 1995). In veterinary medicine, bacitracin has been suggested for the treatment and/or prevention of proliferative adenomatosis in swine (, 1996), swine dysentery (and Froe, 1985), and for clostridial infections in man (Caputo ET AL , 1994) and various animal species (Carman and Wilkins, 1991; Prescott and Baggot, 1993). Two diseases may be of particular interest in this respect, namely proliferative adenomatosis in swine and necrotic enteritis in poultry (Prescott and Baggot, 1993). Recommended dosages for prophylaxis and therapy in poultry are in the range between 50 and 200 ppm, and for swine around 250 ppm (Prescott and Baggot, 1993). The corresponding dosages permitted for growth promotion are between 5 and 100 ppm for poultry, and between 5 and 80 ppm for swine. Growth promoting dosages for calves, lambs and kids are in the range of 5- 80 ppm and for fur animals 5-20 ppm. Bacitracin is widely used for laboratory purposes, in selective media for bacterial culture and in cell culture media. It is also used experimentally as a protease inhibitor (Fukuda ET AL , 1995). The figures in this report is only available in the printed version Tentative sketch of bacitracin B.2 Mode of action and resistance mechanisms Bacitracin inhibits the formation of bacterial cell wall peptidoglycan by complexing directly with the lipid isoprenyl pyrophosphate (IPP) , inhibiting the dephosphorylation reaction that is required for its regeneration. This leads to accumulation of phospholipids inside the cell and inhibition of cell wall formation. Proposed resistance mechanisms include efflux, increased production of IPP kinase, suppression of autolytic systems, reduced membrane permeability and suppressed exopolysaccharide secretion (see below). No information about cross-resistance to other substances has been found. Cross-resistance might be expected in some cases, like when autolytic systems are suppressed. This would be expected to convey cross-resistance to other cell wall inhibitors. Reduced membrane permeability might convey cross-resistance to substances with similar diffusion properties or mechanisms for uptake. It has been claimed that bacitracin "cures" resistance against other antimicrobials and that its use as a feed additive would therefore be purely beneficial from a resistance point of view (, 1978; Gedek, 1981; , 1984; and Wheeler, 1987). However, due to weaknesses in study design in relation to the specific question to be answered, as well as inconsistent results, the only conclusion that can be drawn from these studies is that bacitracin does not seem to induce resistance to any of the other, unrelated, antimicrobials tested. However, as resistance to bacitracin was not determined or characterised, the results provided information about the potential for cross-resistance. Some of the studies mentioned show synergistic effects of bacitracin together with other antimicrobials, but do not investigate any effects on actual resistance genes. Undoubtedly, antimicrobials interfering with cell wall synthesis, such as bacitracin, may act synergistically with other antimicrobials that exert their effects inside the bacterial cell. One must still keep in mind that this most likely has to do with the combined mechanisms of the two antibacterials and does not mean that any resistance genes have been eliminated. B.3 Development of resistance Only two studies that deal specifically with development of bacitracin resistance over time have been found (, 1985; Kaukas ET AL , 1988). These include one experimental study and one field study. In the study by Kaukas ET AL (Kaukas ET AL , 1988), groups of 10 chickens were fed different antibiotics, with one group serving as a non-medicated control group. The bacitracin group was given a dosage of 40 ppm bacitracin in the feed. Enterococci were isolated from cloacal swabs, identified and analysed for antimicrobial susceptibility. When comparing the incidence of bacitracin resistance in enterococci from medicated birds to that in enterococci from non-medicated birds, bacitracin exposure seems to have had effect on the incidence of resistance in %NTEROCOCCUS FAECALIS and % GALLINARIUM. % FAECIUM, however, showed an incidence of resistance to bacitracin as high as in the medicated group, while in the non-medicated group the incidence was . This difference was statistically significant (p<0.005), and indicates that bacitracin in the feed selects for resistant strains of % FAECIUM Further, the incidence of resistance to therapeutic antibiotics, expressed as the Antibiotic Resistance Index () was significantly (p=0.003) higher in all groups receiving antibacterials, as compared to the control group. This increase was most likely associated with an increase in the proportion of % FAECIUM in the enterococcal population of the treated birds. In the publication by (, 1985), the first part is a field survey comparing five commercial premises and a total of nine batches of broiler chickens. One farm, where one batch of birds was reared, used feed with bacitracin at the recommended level for growth promotion. No non-medicated controls were included and no information on whether coccidiostats were used was provided. Cloacal swabs were taken at the beginning and at the end of the rearing period and isolated enterococci were tested for antimicrobial susceptibility. The percentage of strains resistant to bacitracin, among the strains isolated from birds fed bacitracin, increased from to during the rearing period. However, the enterococci were not identified to the species level and no information is provided on how many birds were sampled in each group in this experiment. This, taken together with the fact that there was no non-medicated control group, makes the results hard to evaluate. The second part of the same publication describes an experimental study, where enterococci were also tested for their susceptibility to bacitracin. Bacitracin was not a factor of the experiment but one of the four groups of chickens was a non-medicated control. In this group, the proportion of resistant strains varied between and during the rearing period, which indicates that a day to day variation is not unusual and must be taken into account when evaluating this type of data. Data from different studies on prevalence of resistance to bacitracin have been compiled in table B.I. Unfortunately, most studies do not report values and there seems to be some confusion about what should be the break- point value for different bacteria. Further, the number of strains included are often inadequate for an estimate of the prevalence of resistance. Bearing this in mind, it is noteworthy that the proportion of enterococcal isolates with presumably acquired resistance shows a range from , varying between different animal species and different countries. To fully assess the possible importance of this, information on resistance mechanisms and transferability of resistance in enterococci is needed, as well as consumption figures for different animals and different countries. Resistance to bacitracin in various bacterial species, reported in different studies Devriese et al, 1993Belgium Devriese et al, 1993Belgium et al, 1988 Devriese et al, 1993Belgium Dutta and Devriese, Dutta and Devriese, Dutta and Devriese, Dutta and Devriese, Dutta and Devriese, et al, 1978 Dutta and Devriese, et al, 1978 et al, 1995 et al, 1995 Maple et al, 1989 et al, 1995 et al, 1995 * total number of strains for cattle, poultry and swine = 192 B.4 Acquisition of resistance Published information on bacitracin resistance is sparse, but some hitherto identified mechanisms of resistance to bacitracin are shown in Table B.II. Mechanisms of bacitracin resistance Podlesek et al, 1995 et al, 1993 Pargwette, 1980* Mukherjee et al, Pollock et al, 1994 *this is the mechanism proposed although not proven in the article Podlesek and co-workers (Podlesek ET AL , 1995) characterised a resistance gene, BCR, in "ACILLUS LICHENIFORMIS that codes for proteins forming an ATP- binding transport system in the cell membrane. The proposed action of this transport system is efflux of the bacitracin molecule. Another resistance gene, located on plasmid pXV62, named BACA (, 1993), proposedly encodes a phosphokinase involved in IPP metabolism. A similar enzyme has been characterised in 3TAPHYLOCOCCUS AUREUS (Sandermann Jr and Strominger, 1971), although its possible effects on the bacitracin susceptibility of this bacterium was not investigated. Resistance to bacitracin due to altered cell membrane permeability has been reported (Mukherjee ET AL , 1989). Resistance due to suppressed autolytic enzyme systems has also been suggested (Krogstad and Pargwette, 1980). Suppression of autolytic enzymes makes the bacterium resistant to substances that inhibit peptidoglycan synthesis (, 1970; Krogstad and Pargwette, 1980). Other mechanisms for reduced bacitracin susceptibility that have been suggested include increased production of the , which may competitively overcome the inhibitory effect of bacitracin on peptidoglycan synthesis, and cessation of exopolysaccharide synthesis (Pollock ET AL , 1994). The excretion of polysaccharides require the same that is needed for the synthesis of peptidoglycan and a halt in this excretion will leave more IPP available for cell wall synthesis. This will also occur in the absence of essential components required for exopolysaccharide synthesis, e.g. in an environment depleted of certain sugars (Pollock ET AL , 1994). Transfer of bacitracin resistance seems to have attracted attention from researchers. Transduction between strains of 3TREPTOCOCCUS PYOGENES has been shown to occur (and Ferretti, 1978). However, this study did not investigate what gene(s) and mechanisms were involved in the observed resistance. No other studies, concerning transfer or non-transfer of bacitracin resistance have been found. Bacitracin, like other substances that inhibit late stages in peptidoglycan synthesis, has been shown to induce the expression of the vancomycin resistance gene, VANA in enterococci (and , 1995; Lai and Kirsch, 1996). The practical aspects of this are not clear but it is not likely to be of any clinical importance. B.5 Effects on specific animal diseases Some reports indicate that bacitracin, even at concentrations used for growth promotion, may prevent necrotic enteritis in poultry (Wicker ET AL , 1977; Prescott ET AL , 1978; Stutz ET AL , 1983). Stutz and co-workers (1983) found that supplementing a soybean protein and sucrose-based diet with levels of 5.5, 16.5, or 55 ppm of bacitracin significantly reduced the number of #LOSTRIDIUM PERFRINGENS organisms in the ileal contents of chicks (p<0.05). Prescott and co-workers (1978) reported that inclusion of bacitracin at 200 or /gallon in the drinking water was effective in treating experimentally induced necrotic enteritis in chickens and incorporation of /gallon in the drinking water prevented its occurrence. Wicker and co-workers (1977) found a significant (p<0.01) decrease in mortality due to necrotic enteritis in chickens given 11, 33 or 55 ppm bacitracin in the feed, as compared to non- medicated birds. These studies suggest that bacitracin at the concentrations used for growth promotion also has prophylactic and therapeutic effects on necrotic enteritis in poultry.