Vancomycin Turns Resistant Enterococcus faecium from Cocci to Rods
Vancomycin Turns Resistant Enterococcus faecium from Cocci to Rods
ABSTRACT & COMMENTARY
Synopsis: Exposing vancomycin-resistant Enterococcus faecium to subinhibitory concentrations of the drug inhibits autolysis but not cell-division, resulting in rod-like cells and possible mis-identification of Gram stain.
Source: Lorian V, Fernandes F. The effect of vancomycin on the structure of vancomycin-susceptible and -resistant Enterococcus faecium strains. Antimicrob Agents Chemother 1997;41:1410-1411.
A study of the effect of subinhibitory concentrations of vancomycin on the growth of three susceptible strains of Enterococcus faecium (MICs < 2 mg/L) and three resistant strains (MICs of vancomycin > 200 mg/L and of teicoplanin > 40 mg/L), which presumably carried the vanA resistance gene, showed that the cells divided but failed to separate, resulting in rod-like structures that were twice as long as in susceptible strains. Moreover, the cells were twice as thick as unexposed control cells and contained chains of distinct cells instead of the more usual two separate cells. However, normal cells could easily be produced within five hours of washing the cells to remove the drug and growing them in broth.
n COMMENT BY J. PETER DONNELLY, PhD
This phenomenon of division without separation has been seen by the same group for Staphylococcus aureus exposed to penicillin, which is another of the cell-wall active antibiotics, and suggests that autolysis is inhibited but not cell division. Autolysis is necessary for cell wall production because the matrix has first to be broken before new subunits can be appended, rather like making a stretch limo out of a saloon. First, saw it through the middle, add the extension, then weld it all together again. Like streptococci, enterococci divide in one plane and normally consist of short chains of 2-4 cells. Very rapidly dividing cultures can contain longer chains, but these still appear as a necklace, even when a Gram preparation is examined. However, like streptococci that are exposed to penicillin, these resistant enterococci will appear as Gram-variable misformed rods that can be easily confused with coryneform bacteria. Whether this will be apparent in clinical material remains to be seen. Nonetheless, the microbiologist must now be aware of the possiblity of misdiagnosing enterococcal infection as a "superinfection" with a Gram-positive bacillus, especially as certain coryneformsnotably Corynebacterium jeikeumare multiply resistant and frequently found in the same settings as enterococci. However, they have not, as yet, been reported to be resistant to glycopeptides. Thus, before relaying the results, any putative vancomycin resistant Gram positive coryneform bacillus should first be confirmed by subculture and rechecking the Gram preparation.
Subscribe Now for Access
You have reached your article limit for the month. We hope you found our articles both enjoyable and insightful. For information on new subscriptions, product trials, alternative billing arrangements or group and site discounts please call 800-688-2421. We look forward to having you as a long-term member of the Relias Media community.