Specific Growth Rate Determines the Sensitivity of Escherichia coli to Thermal, UVA, and Solar Disinfection

Abstract

Knowledge about the sensitivity of the test organism is essential for the evaluation of any disinfection method. In this work we show that sensitivity of Escherichia coli MG1655 to three physical stresses (mild heat, UVA light, and sunlight) that are relevant in the disinfection of drinking water with solar radiation is determined by the specific growth rate of the culture. Batch-and chemostat-cultivated cells from cultures with similar specific growth rates showed similar stress sensitivities. Generally, fast-growing cells were more sensitive to the stresses than slow-growing cells. For example, slow-growing chemostat-cultivated cells (D ‫؍‬ 0.08 h ؊1 ) and stationary-phase bacteria from batch culture that were exposed to mild heat had very similar T 90 (time until 90% of the population is inactivated) values (T 90, chemostat ‫؍‬ 2.66 h; T 90, batch ‫؍‬ 2.62 h), whereas T 90 for cells growing at a of 0.9 h ؊1 was 0.2 h. We present evidence that the stress sensitivity of E. coli is correlated with the intracellular level of the alternative sigma factor RpoS. This is also supported by the fact that E. coli rpoS mutant cells were more stress sensitive than the parent strain by factors of 4.9 (mild heat), 5.3 (UVA light), and 4.1 (sunlight). Furthermore, modeling of inactivation curves with GInaFiT revealed that the shape of inactivation curves changed depending on the specific growth rate. Inactivation curves of cells from fast-growing cultures ( ‫؍‬ 1.0 h ؊1 ) that were irradiated with UVA light showed a tailing effect, while for slow-growing cultures ( ‫؍‬ 0.3 h ؊1 ), inactivation curves with shoulders were obtained. Our findings emphasize the need for accurate reporting of specific growth rates and detailed culture conditions in disinfection studies to allow comparison of data from different studies and laboratories and sound interpretation of the data obtained.

Indicator and Pathogen Removal by Low Impact Development Best Management Practices

“…These cultures have substantial physiological differences relative to environmental organisms (such as those in wastewater and stormwater) and often have very different fate and transport characteristics. The culturing methods and media also cause significant differences in microbial attachment efficiency and in die-off rates [118]. In short, a single laboratory growth strain is rarely representative of even the same species of microbial contaminant in real stormwater.…”

Staphylococcus aureus Strain Newman Photoinactivation and Cellular Response to Sunlight Exposure

“…For example, higher concentrations of dissolved oxygen lead to increased photoinactivation of fecal coliforms and Enterococcus faecalis (6,17), and the presence of carotenoid pigments in certain enterococci tends to decrease their susceptibility to photoinactivation (4,18). However, most previous research has focused on loss of culturability as the sole metric of photoinactivation, with a limited number of studies also addressing the effects of sunlight exposure on membrane integrity, cell metabolism, protein content, and gene expression in FIB (6)(7)(8)29). These studies demonstrate that the bacterial photostress response is complex and not yet well understood.…”

“…A number of studies have therefore investigated bacterial photoinactivation in the context of environmental water quality (1)(2)(3)(4)(5)(6)(7), with further studies also focusing on photoinactivation in drinking water treatment (8)(9)(10)(11), photodynamic therapy (12)(13)(14)(15), and wastewater treatment (16)(17)(18)(19)(20). Bacteria can be inactivated by sunlight by direct and indirect mechanisms.…”

Modeling of ultraviolet (UV) radiation under a large pilot-scale designed for wastewater disinfection and inactivation of selected bacteria of Pseudomonas aeruginosa in a laboratory UV device

“…The latency period may also be due to the probability of contact between the disinfectant molecules and microorganisms present in water as conglomerates of different sizes (Mounaouer and Hassen, 2011). The existence of many species of microorganisms and their varying sensitivities to the product used for disinfection may also explain the latency period, which is detected through a comprehensive measure giving an apparent rate of inactivation (Berney et al, 2006).…”