Identify & confirm Staph epidermidis in laboratory ?
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Staphylococcus epidermidis are facultative anaerobes that grow by aerobic respiration or by fermentation and it is the major Coagulase negative (CNS) staphylococcus in human infections.
Most of S. epidermidis are nonpathogenic and they even play a protective role in their host as normal flora. Staphylococcus epidermidis can be a pathogen in the hospital environment.
S. epidermidis cause infection in patients with prosthetic devices and catheters. However diagnosis is little difficult as it is skin contaminant.
When we going to identify this Staph epidermidis  in laboratory , bacteria growth pattern on agar plates gives first clue.
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- When we consider staph epidermidis colony morphology on Blood Agar – they give large(>1 mm), round, non pigmented(white) colonies with entire edge, often without hemolysis.
- When we consider staph epidermidis colony morphology on MacConkey Agar – they give small Lactose Fermenting colonies
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Then we can perform gram test & it gives – Gram positive   cocci in clusters
If gram test gives , gram positive cocci colonies organism should be , Staphylococus , Strptococcus or entero coccus spp
After that , we have to perform catalase test – and staph epidermidis give positive catalase test result.
Remember , if if the organism is gram positive and catalase test gives positivity next test to perform is coagulase test. It helps to identify & differentiate staphylococcus aureus from other staphylococcus species …Â
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Staph epidermidis give negative tube & slide coagulase test results. So according to gram stain ,catalase test & coagulase test results ,in in this stage we can identify this organism as Coagulase negative staph species …
But there few medically important coagulase negative staph species , they are,
- Staph Epidermidis
- Staph Saphrophyticus
- Staph lugdunensis. Â
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S.epidermidis can grow on mannitol salt agar. But it does not ferment mannitol.. so it gives pink colonies in pink medium
Another important test is Novobiocin (5micro gram)susceptibility test. S.epidermidis is sensitive to Novobiocin where as S.saprophyticus is resistant.
Further identification can be done by using commercial kits such as API Staph and using Vitek techniques
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