Question 14 from the second paper of 2017 was the first question to directly ask about this group of bugs. Prior to that the CoNS group had played a fairly peripheral role in the exams, eg. in Question 23.1 from the first paper of 2013 where the candidates were asked how they would react to a S.epidermidis growing from a culture which was drawn from a central line in the process of insertion. For a comprehensive review which goes beyond the requirements of a time-poor exam candidate, one can go to the online article by Davidson and Low at antimicrobe.org.
These cocci are Gram-positive catalase-positive and coagulase-negative. Broadly, you group staphylococci into those which produce coagulase (S.aureus) and those which do not (all others). Coagulase produced by S.aureus enables the conversion of fibrinogen to fibrin, i.e. it facilities blood coagulation. That covers staphylococci in a film of clotted blood, which in turn protects them from phagocytosis and immune detection. This distinction is therefore supposed to be clinically relevant, and the coagulase-negative staphylococci are therefore a low-virulence group because they remain "naked", unable to cloak themselves in this way. However, there is no direct evidence that coagulase is a virulence factor (Foster, 1996) and a few isolates of S.aureus have no coagulase, so this classification system is probably more folklore and tradition rather than science.
This group contains the following relevant members:
In Question 14 from the second paper of 2017, the candidates were asked to contrast the clinical presentation of CoNS and S.aureus infectious processes. Classically, because these organisms are low virulence skin organisms, infections due to them are
In contrast, S.aureus infection tends to progress more rapidly, cause more severe infections, affect relatively healthy people, stimulate a vigorous SIRS response, and occasionally produce a toxic-shock-like superantigen-driven syndrome.
Host factors which favour infection include
Specific infections caused by these organisms:
Blood culture findings suggestive of a true CoNS bacteraemia:
Additionally, there would have to be some sort of judgment call where the interpreter takes the result and considers the risk factors of the patient, i.e. is this patient likely to have a prosthetic device infection? Would such an infection be disastrous?
The Sanford Guide recommends vancomycin as empiric therapy. Most CoNs (80-90%) are resistant to "classical" β-lactams, but sensitive to antistaphylococcal ones like flucloxacillin. Cephazolin and linezolid are alternatives. If a prosthetic device is infected but needs to remain in situ, rifapicin may be used over a long course.
John Jr, Joseph F., R. J. Davidson, and Donald E. Low. "Staphylococcus epidermidis and other Coagulase-Negative Staphylococci." Antimicrobial Therapy 1.
Foster, Timothy. "Staphylococcus." (1996).
Kassis, Christelle, et al. "Differentiating culture samples representing coagulase-negative staphylococcal bacteremia from those representing contamination by use of time-to-positivity and quantitative blood culture methods." Journal of clinical microbiology 47.10 (2009): 3255-3260.
Weinstein, M. P. "Contaminated or not? Guidelines for interpretation of positive blood cultures. WebM&M. January 2008. Agency for Healthcar Research and Quality, Rockville, MD." (2008).
Al-Mazroea, Abdulhadi Hassan. "Incidence and clinical significance of coagulase negative Staphylococci in blood." Journal of Taibah University Medical Sciences 4.2 (2009): 137-147.