Question 12: In the context of infectious disease, which bacterium do you consider most important?

The answer to this question is of course a matter of personal opinion and there is obviously no right or wrong answer. The following essay is only an example of how it may be answered.


The bacterium I have chosen to talk about may not necessarily cause the most number of deaths or have the fastest rate of infection nor is it usually the first bacterium thought of when considering the ‘most scary of bacteria causing infectious disease’ but I feel it is a good example of a class of bacterium that is going to pose a great threat in the future. This class is the SUPERBUG and the bacterium is Staphylococcus aureas, or to be more accurate vancomycin resistant Staphylococcus aureas (VRSA).

S. aureas is a spherical bacterium that is frequently found in the nose and skin of a third of the human population and usually does not cause infection. However it is an opportunistic pathogen and given the chance can cause skin infections such as pimples and carbuncles, scalded skin syndrome or life threatening diseases such as pneumoni, meningitis, osteomyelitis, endocarditis, Toxic shock syndrome (TSS) and septicemia. This organism is also considered to be the cause of the most nosocomial infections reported.

S. aureus was the first bacterium in which penicillin resistance was found—in 1947, just four years after the drug started being mass-produced. When microbes began resisting penicillin, medical researchers fought back with chemical cousins, such as methicillin and oxacillin. By 1953, the antibiotic armamentarium included chloramphenicol, neomycin, terramycin, tetracycline, and cephalosporins. Methicillin was then the antibiotic of choice, but has since been replaced by oxacillin due to significant kidney toxicity. MRSA (methicillin-resistant Staphyloccus aureus) was first detected in Britain in 1961 and is now "quite common" in hospitals. MRSA was responsible for 37% of fatal cases of blood poisoning in the UK in 1999, up from 4% in 1991. Half of all S. aureus infections in the US are resistant to penicillin, methicillin, tetracycline and erythromycin and researchers fear that we may be nearing an end to the seemingly endless flow of antimicrobial drug largely due to the major pharmaceutical companies losing interest in the antibiotics market because of these drugs no longer being as profitable as drugs that treat chronic (long-term) conditions and lifestyle issues

The epidemiology of infections caused by MRSA is rapidly changing. In the past 10 years, infections caused by this organism have emerged in the community and is now an epidemic that is responsible for rapidly progressive, fatal diseases including necrotizing pneumonia, severe sepsis and necrotizing fasciitis. The 2 MRSA clones in the United States most closely associated with community outbreaks, USA400 (MW2 strain, ST1 lineage) and USA300, often contain Panton-Valentine leukocidin (PVL) genes and, more frequently, have been associated with skin and soft tissue infections. Outbreaks of community-associated (CA)-MRSA infections have been reported in correctional facilities, among athletic teams, among military recruits, in newborn nurseries, and among active homosexual men. CA-MRSA infections now appear to be endemic in many urban regions and cause most CA-S. aureus infections

For a long period of time the antibiotic vancomycin was literally considered the drug of last resort however, strains with intermediate (4-8 ug/ml) levels of resistance, termed GISA (glycopeptide intermediate Staphylococcus aureus) or VISA (vancomycin intermediate Staphylococcus aureus), began appearing in the late 1990s. The first identified case was in Japan in 1996, and strains have since been found in hospitals in England, France and the US. In 2002, a newly reported VRSA was isolated from the catheter tip of a renal dialysis patient in Michigan. The isolate contained both the mecA gene (methicillin resistance) and the vanA gene (vancomycin resistance). The presence of the vanA gene was confirmed by polymerase chain reaction and was located on a 60-kb plasmid. The DNA sequence of the VRSA vanA gene was identical to that of a vancomycin-resistant strain of Enterococcus faecalis recovered from the same catheter tip culture. This VRSA was, thus, the first likely transfer in vivo of high-level vancomycin resistance from E faecalis to S aureus. Should this plasmid, or another one like it, be transferred from one S aureus strain to another as rapidly as was the plasmid containing the beta-lactamase gene, this may herald the demise of vancomycin as a clinically useful agent.

When asked to think about the most important bacterium in the context of infectious disease S. aureus may not be the first one that comes to mind, however it is this bacterium and others like it that are at the forefront of the ‘superbug’ emergence. It may be other superbugs such as antibiotic resistant TB that may cause the greater medical challenges in the future but at this moment in time it is bacterium like VRSA that are leading the way in gaining and spreading of antibiotic resistant genes (and have been since the introduction of antibiotics) and therefore they are the most important bacterium in the context of infectious disease at present.