The emergence of <i>Stenotrophomonas maltophilia</i>
Authors: Alan P Johnson, Georgia J Duckworth
Journal: BMJ (2008)

The recent publication of the genome sequence of the bacterium Stenotrophomonas maltophilia was accompanied by a press statement that described this organism as a “newly emerging superbug.”1 2 This was followed by a flurry of headlines in the United Kingdom informing the public of problems related to S maltophilia, such as the apparent “rising death toll fear in hospitals” (Daily Mirror) and the concern of doctors that “no antibiotics can stop it” (the Sun). Suddenly, the public had a new superbug (or even “mega-bug”, as it was referred to in the Sun) to worry about.

So, what must a bacterium do to earn “superbug” status? One prerequisite is resistance to multiple antibiotics and the associated difficulty in treating infections. Using this criterion, S maltophilia is a strong candidate because it is inherently resistant to a wide range of antibiotics and can become more resistant by the acquisition of new genes.3 However, S maltophilia infections are usually treatable because most isolates are susceptible to co-trimoxazole, which by consensus is the drug of choice.3 But it is worrying that effective treatment is limited to a single agent to which resistance has been reported, albeit rarely.

The other characteristic feature of the true superbug is its common occurrence, which reflects the ability to spread easily between patients and even between hospitals. In this regard, it is harder to accept the claim that S maltophilia deserves the superbug epithet because this organism accounts for fewer than 1% of bloodstream infections.4 This is exemplified by comparison with the prevalence of the two pathogens with acknowledged superbug status—meticillin resistant Staphylococcus aureus (MRSA) and Clostridium difficile. Reports to the Health Protection Agency indicated that 4918 cases of bloodstream infection were caused by MRSA in England in 2007,5 compared with 671 reports of S maltophilia.6 Although MRSA and S maltophilia cause other infections besides bacteraemia, the relative occurrence is likely to be similar. These data also contrast with more than 50 000 reported cases of gastrointestinal infections caused by C difficile.5

Outbreaks of MRSA and C difficile are well described and strains of each species with epidemic potential have been identified. In contrast, S maltophilia is of limited pathogenicity and infections in previously healthy patients are unusual. Furthermore, when it is isolated from patients’ specimens, it is often unclear whether this reflects colonisation or contamination. It most often affects patients in critical care units or oncology units, usually those with comorbidities, those who have previously received multiple courses of antimicrobials, or those being ventilated. It may cause long term colonisation in patients with cystic fibrosis. It does not readily spread between patients, and acquisition is usually from environmental sources, particularly moist ones such as water, aqueous solutions, nebulisers, or mechanical ventilators. In these circumstances, it is more deserving of the “opportunist” rather than “superbug” title.

Although in public health terms the burden of disease as a result of S maltophilia is relatively low compared with the many other species of bacteria, fungi, and viruses that cause healthcare associated infections, this is of little consolation to people who develop infections with this organism. So what can be done? Large scale interventions aimed specifically at preventing infection with S maltophilia are unlikely to be initiated because many other pathogens have greater clinical importance in terms of their frequency and the limited therapeutic options. However, many initiatives and interventions aimed at curbing the spread of these other organisms will also affect S maltophilia because the risk factors for colonisation or infection are often similar. Interventions such as improved hospital hygiene, improved attention to line insertion to prevent line associated bacteraemia, and antibiotic stewardship aimed at reducing the pressure for emergence and spread of strains of antibiotic resistant bacteria are all microbiologically non-specific and will affect a wide range of pathogens.7

The current high degree of concern about S maltophilia stemmed from the press release about the characterisation of the organism’s genome. We hope that this new knowledge of the organism’s biology will help allay these concerns by being used to improve diagnostic tests, identify new drug targets, or even develop a vaccine, which could reduce the burden of disease caused by this little known pathogen.