Abstract
The effects of subminimal inhibitory concentrations (sub-MICs) of nine beta-lactam antibiotics [penicillin-G, ampicillin (ABPC), cloxacillin, dicloxacillin, nafcillin, cephalothin, cephradine, cefazolin (CEZ) and cefoxitin] against seven different R-plasmid-bearing gram-negative bacterial strains were tested. Morphological changes, such as filamentous development and lysis, occurred at concentrations considerably lower than the MIC values and the presence of R plasmids did not influence the morphological changes compared with those of Escherichia coli NIHJ, which is non-R-plasmid-bearing. Clear cooperation of ABPC and CEZ with rabbit polymorphonuclear leukocytes PMNs against Pseudomonas aeruginosa and Proteus vulgaris and of CEZ against Salmonella typhimurium was observed, but not against the other strains tested. Therefore, the filamentous bacteria produced as a result of exposure to sub-MICs of antibiotics were not always phagocytosed easily. The ratios between the drug concentrations (microgram/ml) at which the morphological changes began to occur (minimal antibiotic concentration) and the MIC values (microgram/ml) were calculated. A large ratio indicated a wide range of effective concentrations below the MIC value for that antibiotic.
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