Many people have heard of the genus Staphylococcus. The most popular bacteria in this genus are Staphylococcus aureus which is responsible for a multitude of diseases within the human population. One of the more overlooked bacterium from this genus is Staphylococcus epidermidis. This species was originally named Staphylococcus albus by microbiologist Rosenbach in 1884 because of the white colonies they produced in contrast to S. aureus’ yellow colonies.
Staphylococcus epidermidis are gram positive cocci bacteria that are part of the normal flora on the skin and nasal passages. Under a microscope the bacteria will appear in pairs, chains, or clusters. Like most members of this genus, they are non-motile, non spore forming, and are facultative anaerobes. Even though they are facultative anaerobes, not all strains of S. epidermidis will ferment. Another defining feature of the Staphylococcus members is the fact that they are all catalase positive which sets them apart from other gram positive cocci such as Streptococcus or Enterococcus.
S. epidermidis strains are also coagulase negative which easily distinguishes them from any S. aureus bacteria. They are also urease positive, cannot utilize Mannitol, are resistant to the antibiotic Bacitracin, and are sensitive to the antibiotic compound Novobiocin. These are some of the simple tests that can be performed in laboratories to help distinguish S. epidermidis from other Staph species. The colonies are non-hemolytic on SBA (sheep blood agar plates) and are small to medium sized with white coloring.
No special procedures are necessary when collecting for this species but the area must be fully cleansed beforehand to ensure that contamination does not come from the normal flora present on the skin. Selective media can be used to help isolate Staph species such as mannitol salt agar or PEA (phenylethyl alcohol agar). Incubation is normally 18 – 24 hours at 35 – 37o C. Note that even though S. epidermidis is a facultative anaerobe, they grow better in aerobic conditions. When necessary, rapid and automated tests can be used for fast identification of Staphylococcus species even though the results are more accurate when checking for S. aureus, S. epidermidis, and S. saprophyticus.
S. epidermidis is one of the two most clinically relevant coagulase negative Staphylococcus with the other being S. saprophyticus. They are opportunistic pathogens with most infections being acquired in the hospital. S. epidermidis infections are most often found with implants or plastic items that have been implanted in the body. They cause severe urinary tract infections from indwelling catheters. The bacteria are also responsible for many cases of Endocarditis resulting from prosthetic heart valve implants. The mortality rate for S. epidermidis resulting Endocarditis is about 70%. Septicemia is also a very large concern when intravascular catheters are in use. The reason why S. epidermidis is such a problem in these cases is because the bacteria are capable of forming bio-films on plastic. This makes it very important for aseptic techniques to be followed at all times.
Infections are more prevalent and severe in patients that are immuno-compromised. These patients include the elderly, very young, cancer patients, etc. One of the biggest concerns with S. epidermidis is that because nosocomial infections are so prevalent, antibiotic resistance is increasing very quickly. Just like its counterpart S. aureus has MRSA strains, there are now MRSE (Methicillin resistant staphylococcus epidermdis) strains popping up. Vancomycin is still the best course of treatment for people fighting MRSE infections.
Sources
Mahon, C. R., Lehman, D. C., & Manuselis, G. (2007). Textbook of diagnostic microbiology (3rd ed.). St. Louis, Mo.: Saunders Elsevier.
