Seasonality of the antibiotic resistance gene bla CTX-M in temperate Lake Maggiore

The beta lactamase gene bla, responsible of the resistance to cephalosporins, has been detected in microbes from hospitals to open waters. We studied the seasonality and stability of bla in Lake Maggiore over 3 years and the role of potential inputs of allochthonous bacteria and/or antibiotic pollution in promoting its occurrence. bla was mainly present from January to July in the pelagic microbial community and the gene occurrence was significantly related to low water temperature. To evaluate its temporal stability in the bacterial community over a short period, we measured bla daily over the course of 6 days. The gene was below the limit of quantification except for one sampling when its abundance peaked, suggesting a point contamination. The bacterial community of the lake in which bla was detected suggests that at least two distinct bacterial populations contained the gene. The occurrence of known bla containing genera and the occurrence of the gene, however, did not overlap. Furthermore, the experimental addition of cefotaxime to lake water incubations did not promote abundance of the gene. These data imply that bla was present in the environmental microbial community. Increases of gene abundances were likely caused by environmental parameters other than antibiotic contamination.