Sunlight Parameters Influence the Survival and Decline of Salmonella and Escherichia coli in Water

Abstract

The effect of variations in temperature, UV radiation, and sunlight intensity on Escherichia coli, E. coli O157:H7, Salmonella Newport, and antibiotic resistant (ABR) variants of E. coli O157:H7 and Salmonella Newport exposed to sunlight was evaluated. Bacterial strains suspended in sterile deionized water at a concentration of 8 log CFU/mL were exposed to sunlight on three different days for 180 min; control treatments were stored in the dark. The mean temperature of 30.08 and 26.57°C on day 1 and day 3, respectively, was significantly different (P < 0.05). The UV intensity was significantly different on all 3 days, and sunlight intensity significantly differed on day 3 (P < 0.05). Bacterial population decline positively correlated with temperature, sunlight, and UV intensity. Differences in bacterial population declines differed among species, ABR profile, and day of exposure (P < 0.05). On day 1 and day 2, the populations of E. coli dropped below the limit of detection (1 log CFU/mL), whereas the percentage of live cells was 67 and 6.6%, respectively. The artificial neural network model developed to predict bacterial survival under different environmental conditions suggested that Salmonella cells were more resistant than E. coli cells. The ABR strains had significantly higher numbers of viable cells after sunlight exposure (P < 0.05). Sunlight-exposed cells resuscitated in tryptic soy broth varied in maximum population density and maximum specific growth rate based on bacterial species and presence of ABR. Morphological changes such as viable but nonculturable state transition and filament formation were detected in subpopulations of sunlight-exposed bacteria. Daily fluctuations in UV and sunlight intensity can result in significant variations in bacterial decline and recovery.