In vitro studies on bacterial colonization. The effects of beta-lactam antibiotics on the growth of Escherichia coli and Pseudomonas aeruginosa in mixed culture.

Many cases of Pseudomonas aeruginosa infection are considered to be secondary superinfections, resulting from bacterial colonization. Such cases of superinfection with P. aeruginosa developing after administration of cephalosporin or penicillin are offering serious clinical problems. To make a fundamental analysis of the development of such superinfections, attempts were made to compare the growth patterns of Escherichia coli and P. aeruginosa in pure and mixed cultures and to determine the effects of cephalothin, cefazolin, cephalexin, and ampicillin on the growth patterns. In mixed cultures, the growth of P. aeruginosa was markedly inhibited by E. coli. The higher the concentration of each of the cephalosporins and ampicillin added to the mixed culture, the smaller the population of E. coli sensitive to these agents. When the population of E. coli became smaller than that of P. aeruginosa, which is resistant to these agents, the latter was restored to the same population level as that in pure cultures. Experimental bacterial colonization, by which the predominant population of E. coli was replaced by that of P. aeruginosa in mixed culture, was brought about more efficiently with the cephalosporins than with ampicillin. This might be accounted for by the difference in minimal inhibitory concentration for P. aeruginosa between ampicillin and the other three agents.