Abstract
Horizontal gene transfer (HGT) of antibiotic resistance genes has received increased scrutiny from the scientific community in recent years owing to the public health threat associated with antibiotic resistant bacteria. Most studies have examined HGT in growing cultures. We examined conjugation in growing and non-growing cultures of E. coli using a conjugative multi antibiotic and metal resistant plasmid to determine physiochemical parameters that favor horizontal gene transfer. The conjugation frequency in growing and non-growing cultures was generally greater under shaken than non-shaken conditions, presumably due to increased frequency of cell collisions. Non-growing cultures in 9.1 mM NaCl had a similar conjugation frequency to that of growing cultures in Luria-Bertaini broth, whereas those in 1 mM or 90.1 mM NaCl were much lower. This salinity effect on conjugation was attributed to differences in cell-cell interactions and conformational changes in cell surface macromolecules. In the presence of antibiotics, the conjugation frequencies of growing cultures did not increase, but in non-growing cultures of 9.1 mM NaCl supplemented with Cefotaxime the conjugation frequency was as much as nine times greater than that of growing cultures. The mechanism responsible for the increased conjugation in non-growing bacteria was attributed to the likely lack of penicillin-binding protein 3 (the target of Cefotaxime), in non-growing cells that enabled Cefotaxime to interact with the plasmid and induce conjugation. Our results suggests that more attention may be owed to HGT in non-growing bacteria as most bacteria in the environment are likely not growing and the proposed mechanism for increased conjugation may not be unique to the bacteria/plasmid system we studied.
Highlights
Antibiotic resistance has become a serious public health issue (de Kraker et al, 2011; Smith and Coast, 2013) and the spread of antibiotic resistant genes (ARG) via horizontal gene transfer (HGT) is a topic that has received increased scrutiny by the scientific community in recent years
The observation was generally true for cultures supplemented with antibiotics with the notable exception of non-shaken cultures in 9.1 mM NaCl supplemented with 25 μg/ml of Cefotaxime Sodium Salt (CFX) which had much higher conjugation frequencies (Table 1, Figures 2, 3)
At 22◦C, conjugation only occurred in LB cultures without antibiotics, LB supplemented with 25 μg/ml of CFX, and in the 9.1 mM NaCl shaken cultures without antibiotics (Table 1)
Summary
Antibiotic resistance has become a serious public health issue (de Kraker et al, 2011; Smith and Coast, 2013) and the spread of antibiotic resistant genes (ARG) via horizontal gene transfer (HGT) is a topic that has received increased scrutiny by the scientific community in recent years. Many studies have examined conjugation at 37◦C (Khalil and Gealt, 1987; Hayashi et al, 1988; FernandezAstorga et al, 1992; Sandegren et al, 2012; Zhang et al, 2013; Moller et al, 2017), while many other investigators have utilized other temperatures for their experiments Many studies have examined conjugation at 37◦C (Khalil and Gealt, 1987; Hayashi et al, 1988; FernandezAstorga et al, 1992; Sandegren et al, 2012; Zhang et al, 2013; Moller et al, 2017), while many other investigators have utilized other temperatures for their experiments (Fernandez-Astorga et al, 1992; Muela et al, 1994; Beuls et al, 2012; Jutkina et al, 2016; Lopatkin et al, 2016; p. 74; Jutkina et al, 2018)
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