Amensal indirect interactions between a Klebsiella pneumoniae microcin-producing strain and several Escherichia coli strains, all of intestinal origin, were studied. Mixed batch cultures of both microcin-producing and microcin-sensitive strains showed that microcin production and excretion into the medium allowed the producer strain to prevail over sensitive strains, even when initial competition conditions were highly unfavourable for the producer. Mixed cultures also showed the production of a microcin-antagonist by the same microcin-producing strain when the nutrients in the medium had been depleted. The antagonist apparently promoted the viability of sensitive cells already damaged by microcin. These results have likely ecological implications.

Antagonism between fosfomycin and antipseudomonal penicillins, cefotaxime, and ceftriaxone was observed in Pseudomonas aeruginosa RYC212. Fosfomycin, a non-i-lactam antibiotic that acts on bacterial cell wall synthesis, decreased the expression of penicillin-binding protein 3 and induced j8-lactamase. The antagonistic effect was reduced in the presence of high concentrations of the ,l-lactamase inhibitor tazobactam or in fosfomycin-resistant mutants. We suggest that products resulting from fosfomycin cell wail damage could interact with a system that regulates penicillin-binding protein and I8-lactamase production.

Microcin D93 is an antibiotic substance produced by Escherichia coli strains which harbor the 5.5-kilobase plasmid pMccD93. Its production is unaffected by the use of different carbon and ammonia sources, different phosphate concentrations, or mitomycin C. We developed a method for purifying this microcin based on gel permeation chromatography and reverse-phase high-pressure liquid chromatography. The antibiotic appears to be a small, hydrophilic, basic peptide, active on E. coli and Proteus, Citrobacter, and Pseudomonas species and much more active on recA strains than on their isogenic wild type. Diminution of the rate of DNA biosynthesis without any apparent effect on other macromolecules appears to be a primary effect in the action of microcin D93.

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