Competition assays between ESBL-producing E. coli and K. pneumoniae : an additional cost on fitness

Abstract

Enterobacteriacae are found ubiquitous in nature and constitute a major part of the human gut normal flora. In specific cases, these bacteria can cause opportunistic infections such as urinary tract infections, blood infections, etc. The emergence of resistance in these organisms is known to increase the severity of infections and cause a threat to the patient. Many -lactamase-producing bacteria belong to the Enterobacteriacae family such as, Escherichia coli and Klebsiella pneumoniae. In previous work (Challita et al., 2017), we have shown that the expression of acquired -lactamase gene comes on the expense of the bacterial fitness. As a result, bacteria become less competitive vis-à-vis its sensitive counterpart when cultured in an antibiotic free environment. The present study aims to evaluate the growth of extended-spectrum -lactamase (ESBL)-producing Enterobacteriacae: E. coli and K. pneumoniae, when present in competition. In-vitro interspecies competition assays were conducted in an antibiotic-free medium; sensitive (S) and ESBL-producing (R) bacteria were incubated at 37°C alone and in combination. In parallel, the OD580 was measured at each time point for all the cultures. The assays were repeated using different E. coli and K. pneumoniae strains in 14 different combinations. The results showed that K. pneumoniae (R) presents a fitness cost possibly linked to the acquisition of resistance gene, when competing against E. coli (S). On the other hand, E. coli (R) shows a fitness advantage when growing in presence of one K. pneumoniae (S) whereas it presents a reduced fitness in presence of two sensitive strains. In addition, competition assays conducted between sensitive strains and others between ESBL-producing strains were used as control. These results suggest that ESBL-production genes in E. coli and K. pneumoniae may confer a fitness cost that leads to the decrease in frequency of these bacteria in interspecies competitions. These findings are consistent with previous studies consisting of intra-species interactions. Culturing bacteria in a medium with more diverse isolates can provide better insights into bacterial competition and resistance dynamics, which can be exploited in the search for alternative therapeutic approaches towards the colonization of resistant bacteria.