Decrease in Ribosomal Density of Proteus mirabilis Exposed to Subinhibitory Concentrations of Ampicillin or Cephalothin

By conventional light microscopy, it has been shown that gram-negative bacilli exposed to subinhibitory concentrations of benzyl-penicillin become very much elongated and appear as filaments and whorls (1-5). Although it had been suggested that cephalosporins have the same modes of action as do penicillins (6), it has been shown (7, 8) that at subinhibitory concentrations penicillins cause a remarkable elongation (up to 93 μm), whereas cephalosporins cause globules and a minimal cell elongation of Proteus mirabilis (up to 14 μm).

The present study was undertaken to determine whether transmission electron microscopy would reveal other morphological differences in the effects produced by subinhibitory concentrations of a penicillin (ampicillin) and a cephalosporin (cephalothin) on P. mirabilis 6.

Materials and Methods. Exposure to antibiotics. P. mirabilis 6, originally isolated from the urine of a patient at Bronx-Lebanon Hospital, was grown in trypticase soy broth (TSB) (BBL) for 18 hr at 37°. The culture was diluted 1:100 in TSB and 0.1 ml was spread on a filter membrane PHWP09025 (Millipore Corp., Bedford, MA) and placed on the surface of trypticase soy agar (TSA) (BBL) in a petri dish. The plates with the inoculated membranes were incubated at 37° for 90 min, after which membranes were removed and transferred onto other TSA plates containing cephalothin (Lilly) or ampicillin (Bristol) at various concentrations and incubated for another 180 min. The antibiotic concentrations were equal to the minimum inhibitory concentration (MIC) as determined by an agar dilution technique (9) or to 2-fold dilutions of each MIC down to 1:16 MIC. The MIC's were 1.56 μg/ml for ampicillin and 3.1 μg/ml for cephalothin.