Effects of Sublethal Concentrations of Benzylpenicillin on Pseudomonas aeruginosa

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Pseudomonas aeruginosa produces a low basal level of β-lactamase (0.002 to 0.004 IU/mg of protein when benzylpenicillin is used as substrate). The β-lactamase specific activity can be increased several hundredfold by growing the bacteria in the presence of β-lactam antibiotics. This induction was studied in Pseudomonas aeruginosa 1822s. The single-cell resistance to benzylpenicillin was 750 μg/ml. In liquid culture all concentrations of benzylpenicillin tested (25 to 2,000 μg/ml) affected the bacteria similarly: β-lactamase formation was induced, the cells became cholate sensitive, growth rate decreased, filaments were formed, and β-lactamase was excreted. The effect appeared earlier the higher the concentration of the antibiotic. Most of the effects obtained are concerned with the functioning of the outer membrane. The excretion of β-lactamase seems to be due to an opening of the periplasmic volume rather than to lysis of the cells. Carbenicillin gave the same effects as benzylpenicillin at the same concentrations; the 10-fold lower resistance to carbenicillin than to benzylpenicillin can be explained by the inability of the inducible β-lactamase to hydrolyze carbenicillin. The induced β-lactamase was first cell bound and to a great extent located in the periplasmic volume, but later it was excreted into the medium. This extracellular activity was responsible for the detoxification of the medium. This is analogous to the behavior of gram-positive bacteria rather than to that of Enterobacteriaceae.

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