The next day the blood cultures are positive and a Gram-positive coccus in clumps is seen in the Gram film (this is the most common appearance of a positive blood culture) but what does this “gobbledygook” terminology mean?
Gram-positive cocci that form clumps in blood cultures are always Staphylococci (except for a few oddities only Microbiologists really need worry about!). Staphylococci are most frequent because 1) they are a common cause of infection 2) they are the most common skin contaminants. So how do you tell a contaminant from an infection? As a general rule Coagulase-negative Staphylococci are likely to be contaminants whereas Staphylococcus aureus is almost always significant. Still none the wiser about how to tell them apart? Read on...
What do Microbiologists mean when they say a Staphylococcus is Coagulase-negative? Well, essentially what this means is that the Staphylococcus is not Staphylococcus aureus; as Staphylococcus aureus is “Coagulase-positive”. The term “Coagulase-negative Staphylococcus” often shortened to CoNS, would perhaps be more helpfully described as “Staphylococcus sp. which is not Staphylococcus aureus” rather than Coagulase-negative Staphylococcus. So where did this “gobbledygook” terminology come from?!
- Coagulase (an enzyme that coagulates serum) result: Staphylococcus aureus is Coagulase positive (i.e. not a Coagulase-negative Staphylococcus)
- DNAse (an enzyme that breaks down DNA) result: Staphylococcus aureus is DNAse positive
- Clumping factor (commonly known as slide coagulase...but this is an entirely different test to Coagulase) result: Staphylococcus aureus produces clumping factor
We still revert to using these tests when the machines aren’t working for some reason.
- CoNS rarely cause disease and when grown from blood cultures more usually represent contamination from the skin
- Staphylococcus aureus is rarely a contaminant and causes serious infections e.g. sepsis
So are CoNS insignificant then?
There are lots of CoNS, some are more commonly isolated in microbiological specimens than others, the list below is not exhaustive. CoNS occasionally cause infection especially on intravascular devices but CoNS rarely make a patient septic.
Most frequently associated with human disease:
- S. epidermidis (the most common CoNS and usually a contaminant)
- S. lugdenensis (occasional cause of infective endocarditis)
- S. haemolyticus (no specific site of infection)
- S. saprophyticus (can cause urinary tract infections, commonly seen in pregnancy)
Rarely associated with human disease:
- S. hominis
- S. capitis
- S. pasteuri
- S. saccharolyticus
- S. simulans
- S. warneri
- S. schleiferi
CoNS contamination can occur with a breakdown in aseptic technique either because of poor technique or with difficult patients who are agitated or confused and who are moving around whilst you are trying to get the blood sample. Another reason for contamination is if there is a contraindication to the use of chlorhexidine to sterilise the skin such as in neonates and those with skin conditions such as eczema.
If a patient has a Staphylococcus aureus bacteraemia then the most likely sources of infection are:
- Skin (duration of treatment: 2 weeks)
- Bone (duration of treatment: 6 weeks)
- Joint (duration of treatment: 6 weeks)
- Heart (duration of treatment: 4-6 weeks)
- Intravascular devices e.g. cannula, central venous catheter (duration of treatment: 2 weeks)