[Formaldehyde resistance to gram-negative aerobic rods from municipal sewage water]

In a municipal sewage works, a total of 30 sewage samples (19 from the inlet and 11 from the outlet of the sewage works) were analyzed for the quantitative and orientative qualitative content of microorganisms. With an incidence peak of the total bacterial count of greater than or equal to 1 X 10(6) cfu/ml in the inlet, both samples showed bacterial contents of 1 to 9 X 10(5) cfu/ml with use of MC agar and Endo agar. Fuchsin glistening colonies as well as the total bacterial counts on Sabouraud agar and Leifson agar as well as on kanamycin-esculin agar showed frequency peaks which were one power of ten lower. For pathogenic staphylococci, a distribution below the limit of detection (down to 9 X 10(3) cfu/ml) was found, and in a single case in the region of 10(4) cfu/ml. As a rule, the samples drawn from the outlet of the municipal sewage works showed values which were one power of ten lower than in the inlet. With admixture of formaldehyde in bacteriostatically active concentrations, colonies could still be cultured to a small extent with a final concentration of 0.03 weight % HCHO in the inlet, whereas values were below the limit of detection in all outlet samples. The differentiated Gram-negative aerobic rods were mainly representatives of the species Pseudomonas aeruginosa and Pseudomonas putida. In a modified quantitative suspension trial, only occasional Gram-negative rods in the inlet samples survived 30 min exposure to 0.5 weight % HCHO. In comparison, the results of the outlet samples showed a very much lower resistance to formaldehyde. The 11 strains isolated under these conditions could be classified under six species, including four Pseudomonas aeruginosa strains. Gram-negative rods which had previously tolerated a concentration of 0.12 weight % formaldehyde in the bacteriostasis revealed a raised resistance to formaldehyde only to an inappreciable extent under the conditions of the qualitative suspension trial. Overall, enhancements of resistance to formaldehyde such as we observed in isolates from hospital sewage could not be detected in bacterial strains isolated from the inlet or outlet of a municipal sewage works. The investigations carried out thus do not indicate that a quantitatively significant increase or a persistence of the resistance to formaldehyde is to be expected outside the actual hospital environment.