Ultrastructural analysis of slime positive & slime negative Staphylococcus epidermidis isolates in infectious keratitis.

BACKGROUND & OBJECTIVE Slime is a major determinant of Staphylococcus epidermidis adherence. The established methods of laboratory detection of slime production by this organism i.e., Christensen's tube method and congo red agar plate method, can both yield inconclusive and/or intermediate results. We, therefore tried to find out electronmicroscopically the localization of slime in relation to the bacterial cell wall and look for the effect, if any of the slime location on the staphylococcal adherence as well as on the quantum of slime production. METHODS A total of 132 coagulase negative staphylococci from cases of infectious keratitis were identified as S. epidermidis following the recommended protocol. Slime was detected both by Christensen's tube method and congo red agar plate method. Antibiotic sensitivity testing was performed by standardized disc diffusion method. Adherence of the organisms to artificial surfaces was determined by a quantitative method and transmission electron microscopy was carried out by the conventional techniques. RESULTS Of the total 132 isolates, 57 (43.2%) were slime positive and 75 (56.8%) were slime negative. Twenty seven (47.4%) of the 57 slime producing organisms were multi drug resistant as compared to only 12 (16%) of 75 nonslime-producing organisms (P<0.001). A majority i.e., 45 (78.9%) of 57 adherent organisms were slime producers as against 12 (16%) of 75 nonadherent organisms. Electron microscopic study revealed a thick viscid layer of slime anchoring to the bacterial cell wall, especially in adherent organisms and those yielding positive slime test. Some of the organisms showed loose nonadherent slime and those were mostly nonadherent to artificial surfaces. INTERPRETATION & CONCLUSION Slime and multi drug resistance were the important virulence factors of S. epidermidis in bacterial keratitis. It was the adherent slime (i.e., slime in intimate association with the bacterial cell wall as shown by electron microscopy) only, which was responsible for resistance to multiple antibiotics and for the adhesion phenomenon observed in the quantitative slime test.