[Epidemiology of microbial resistance to biocides]

In order to estimate the distribution of bacteria and fungi with an elevated level of resistance to antimicrobial substances, we have analyzed water samples and surveyed institutions presumably concerned with analyses of microbial resistance (university institutes for hygiene, health authorities) by means of questionnaire. A total of 41 water samples was drawn from various aquatic biotopes in the region of Heidelberg. The samples originated from the effluents of a community sewage treatment plant, from the Neckar river, from drinking water supplies and from public swimming pools. The following substance groups were included in a search for bacteria with an elevated resistance to antiseptics and disinfectants: Aldehydes, biguanides, quaternary ammonium compounds (QAC), phenols and halogen. Upon determination of the maximal tolerated concentrations to these antimicrobial agents, samples of treated wastewater effluents showed a considerably higher prevalence of bacteria resistant to formaldehyde, chlorhexidine and QAC. The highest levels of resistance were found in Pseudomonas aeruginosa, Pseudomonas putida, Pseudomonas fluorescens and Enterobacter species. The maximal tolerated level of 0.06 percent formaldehyde by weight was considerably higher than the levels tolerated by the corresponding ATCC reference strains. The highest levels of resistance to a biguanid/QAC preparation were seen in isolates of Alcaligenes species and Providencia species, with a maximal tolerated level of 0.5 volume percent. In addition, several isolates of E. coli and Klebsiella species were observed which showed a considerably higher level of resistance to the biguanide preparation as compared to the corresponding ATCC reference strains. A questionnaire was mailed to a series of institutions and companies presumably concerned with problems of microbial resistance. Responses were in accordance with our own observations, both with regard to the spectrum of bacteria observed and to the antimicrobial substances concerned.