Cultures ofStaphylococcus aureus, Neisseria gonorrhoeae, and Streptococcus bovis were incubated on membrane filters on agar containing antibiotics at onethird to one-fourth the minimal inhibitory concentration for the particular bacterial strain. S. aureus was grown in the presence of oxacillin, cephaloridine, or lincomycin. N. gonorrhoeae and S. bovis were grown in the presence of penicillin. The membranes were then incubated in drug-free agar, after which viability was determined and the cells were examined by electron microscopy. S. aureus exposed to oxacillin and cephaloridine grew into cells two to seven times larger than normal that contained thick multiple cross walls. S. aureus exposed to lincomycin grew into cells 1.5 to 2 times larger than normal, with multiple thick cross walls and periheral cell walls twice the normal thickness. N. gonorrhoeae cells exposed to penicillin were slightly larger than normal and had cross walls that were up to eight times thicker than normal. After transfer to drugfree agar, cells became smaller, and some normal organisms could be seen. S. bovis incubated in the presence of penicillin grew into filaments that contained no cross walls. Two hours after the return to drug-free agar, filaments with cross walls as well as normal cells were observed. Exposure to subinhibitory concentrations of penicillin did not affect the growth of the peripheral cell wall of S. aureus, N. gonorrhoeae, or S. bovis, but appeared to inhibit lysis of cross walls in S. aureus and N. gonorrhoeae and to inhibit the synthesis of cross walls in S. bovis; that is, the rates of peripheral and cross wall formation differed in their susceptibility to penicillin. These facts suggest that peripheral growth and cross wall formation in cocci are separable processes.During bacterial growth there is an equilibrium between synthesis and lysis within the cell wall (24, 29). Cross wall growth at the equatorial plane is probably the only site of cell wall growth in gram-positive cocci (1, 5). During division, the cell wall of cocci grows inward and eventually forms a transverse cross wall (septum).Cross wall cleavage before cell separation is carried out by one or more autolytic enzymes (16,17,31,33). Subinhibitory concentrations of penicillin have appeared to inhibit cross wall lysis in Staphylococcus aureus (22). This paper reports changes in cross wall structure in three cocci species grown in the presence of subinhibitory concentrations of various antibiotics.MATERIALS AND METHODS Strain FDA 209P (ATCC 6538P) ofStaphylococcus aureus and a clinical isolate of Streptococcus bovis (8) were grown in Trypticase soy broth (BBL) for 20 h at 37 C. A clinical isolate ofNeisseria gonorrhoeae (21) was grown on brain heart agar (Difco) with 5% horse blood and IsoVitalex (BBL) (BHASI) for 24 h at 37 C in a 10% CO2 atmosphere.Trypticase soy agar (BBL) and BHASI plates with and without antibiotics were prepared. Penicillin G (Squibb), oxacillin (Bristol), cephaloridine (Lilly), and lincomycin (Upjohn) were used. Drug concentr...

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Subinhibitory concentrations of antibiotics can produce in vitro aberrant forms of bacteria that are similar to those observed in specimens and cultures from patients being treated with antibacterial agents. Eight species of bacteria were grown on membranes placed on agar containing subinhibitory concentrations of nine antibiotics. The resulting organisms were examined by Gram stain and electron microscopy. Gram stains showed filamentous and granular forms of enterobacteria with bipolar staining, giant staphylococci, and rodlike pneumococci. Electron micrographs showed changes in the number and distribution of ribosomes in enterobacteria and septum abnormalities in cocci. Such abnormal forms can occasionally simulate the appearance of quite different species, and they may indicate the presence of a subinhibitory antibiotic concentration at the site of infection as a result of prior antibacterial therapy.

Candida glabrata is an emerging opportunist pathogen in immunosuppressed patients. C. glabrata is resistant to many antifungal agents and until recently, there have been no standard treatment regimens for this organism. A mouse model was established using mice immunosuppressed with 5 fluorouracil to evaluate amphotericin B, flucytosine, fluconazole and their combinations to treat an intravenously induced C. glabrata infection. Treatment with fluconazole, flucytosine, amphotericin B or a combination was begun one day after infection. Following 5 days of treatment, the mice were killed for fungal counts in kidneys and spleen. At the doses used, amphotericin B was superior to fluconazole or flucytosine alone in the treatment of C. glabrata infections. Flucytosine reduced the fungal burden in the kidney for only two of four isolates of C. glabrata. The combination of fluconazole and flucytosine was superior to these agents alone in reducing the tissue burden in the kidney for one isolate of C. glabrata. High doses of fluconazole alone produced modest reductions in kidney counts but did not reduce spleen tissue counts. There was poor correlation between in-vitro MICs and in-vivo results.