Effect of growth phase on the Escherichia coli response to ultraviolet-A radiation: influence of conditioned media, hydrogen peroxide and acetate

The Effects of Freeze-Thaw and UVC Radiation on Microbial Survivability in a Selected Mars-like Environment

The purpose of this study was to determine survivability of Escherichia coli, Deinococcus radiodurans and Paraburkholderia fungorum under Mars-simulated conditions for freeze-thawing (−80 °C to +30 °C) and UV exposure alone and in combination. E. coli ATCC 25922, D. radiodurans and P. fungorum remained viable following 20 successive freeze-thaw cycles, exhibiting viabilities of 2.3%, 96% and 72.6%, respectively. E. coli ATCC 9079 was non-recoverable by cycle 9. When exposed to UV irradiation, cells withstood doses of 870 J/m² (E. coli ATCC 25922), 200 J/m² (E. coli ATCC 9079), 50,760 J/m² (D. radiodurans) and 44,415 J/m² (P. fungorum). Data suggests P. fungorum is highly UV-resistant. Combined freeze-thawing with UV irradiation showed freezing increased UV resistance in E. coli ATCC 25922, E. coli DSM 9079 and D. radiodurans by 6-fold, 30-fold and 1.2-fold, respectively. Conversely, freezing caused P. fungorum to exhibit a 1.75-fold increase in UV susceptibility. Strain-dependent experimentation demonstrated that freezing increases UV resistance and prolongs survival. These findings suggest that exposure to short wavelength UV rays (254 nm) and temperature cycles resembling the daily fluctuating conditions on Mars do not significantly affect survival of D. radiodurans, P. fungorum and E. coli ATCC 25922 following 20 days of exposure.

How to Perform a Microfluidic Cultivation Experiment—A Guideline to Success

As a result of the steadily ongoing development of microfluidic cultivation (MC) devices, a plethora of setups is used in biological laboratories for the cultivation and analysis of different organisms. Because of their biocompatibility and ease of fabrication, polydimethylsiloxane (PDMS)-glass-based devices are most prominent. Especially the successful and reproducible cultivation of cells in microfluidic systems, ranging from bacteria over algae and fungi to mammalians, is a fundamental step for further quantitative biological analysis. In combination with live-cell imaging, MC devices allow the cultivation of small cell clusters (or even single cells) under defined environmental conditions and with high spatio-temporal resolution. Yet, most setups in use are custom made and only few standardised setups are available, making trouble-free application and inter-laboratory transfer tricky. Therefore, we provide a guideline to overcome the most frequently occurring challenges during a MC experiment to allow untrained users to learn the application of continuous-flow-based MC devices. By giving a concise overview of the respective workflow, we give the reader a general understanding of the whole procedure and its most common pitfalls. Additionally, we complement the listing of challenges with solutions to overcome these hurdles. On selected case studies, covering successful and reproducible growth of cells in MC devices, we demonstrate detailed solutions to solve occurring challenges as a blueprint for further troubleshooting. Since developer and end-user of MC devices are often different persons, we believe that our guideline will help to enhance a broader applicability of MC in the field of life science and eventually promote the ongoing advancement of MC.

Growth conditions influence UVB sensitivity and oxidative damage in an estuarine bacterial isolate

The dose-dependent variation of oxidative cellular damage imposed by UVB exposure in a representative estuarine bacterial strain, Pseudomonas sp. NT5I1.2B, was studied at different growth phases (mid-exponential, late-exponential, and stationary), growth temperatures (15 °C and 25 °C) and growth media (nutrient-rich Tryptic Soy Broth [TSB] and nutrient-poor M9). Survival and markers of oxidative damage (lipid peroxidation, protein carbonylation, DNA strand breakage, and DNA-protein cross-links) were monitored during exposure to increasing UVB doses (0-60 kJ m(-2)). Oxidative damage did not follow a clear linear dose-dependent pattern, particularly at high UVB doses (>10 kJ m(-2)), suggesting a dynamic interaction between damage induction and repair during irradiation and/or saturation of oxidative damage. Survival of stationary phase cells generally exceeded that of exponential phase cells by up to 33.5 times; the latter displayed enhanced levels of DNA-protein cross-links (up to 15.6-fold) and protein carbonylation (up to 6.0-fold). Survival of mid-exponential phase cells was generally higher at 15 °C than at 25 °C (up to 6.6-fold), which was accompanied by lower levels of DNA strand breaks (up to 4000-fold), suggesting a temperature effect on reactive oxygen species (ROS) generation and/or ROS interaction with cellular targets. Survival under medium-high UVB doses (>10 kJ m(-2)) was generally higher (up to 5.4-fold) in cells grown in TSB than in M9. These results highlight the influence of growth conditions preceding irradiation on the extent of oxidative damage induced by UVB exposure in bacteria.

Effects of cell surface loading and phase of growth in cold atmospheric gas plasma inactivation of Escherichia coli K12

AIMS

To determine the effects of surface cell concentration and phase of growth on the inactivation of Escherichia coli cells using an atmospheric nonthermal plasma.

METHODS AND RESULTS

Cells of E. coli K12 were deposited onto the surface of membrane filters and exposed to the plume from a cold atmospheric gas plasma. Scanning electron microscopy revealed severe loss in structural integrity of plasma-treated cells, and optical emission spectra indicated that inactivation was brought about by reactive plasma species. The survival of E. coli cells was found to depend on the cell surface density: as the surface density increased from 10(7) to 10(11) CFU cm(-2), the rate constant in the Baranyi inactivation model decreased from 19.59 to 1.03 min(-1). Cells harvested from mid-exponential, late exponential and stationary phases of growth displayed differences in their resistances to the effects of the plasma however, exponential phase cells were not more susceptible than those from the stationary phase.

CONCLUSIONS

High surface concentrations of cells affects the penetration of plasma species and treatment effectiveness. The physiological state of cells, as determined by phase of growth, affects their resistance to plasma inactivation.

SIGNIFICANCE AND IMPACT OF THE STUDY

In designing inactivation treatments, surface concentration and cell physiology need to be taken into account.

Specific growth rate determines the sensitivity of Escherichia coli to thermal, UVA, and solar disinfection

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 mu 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 (mu = 1.0 h(-1)) that were irradiated with UVA light showed a tailing effect, while for slow-growing cultures (mu = 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.

Effects of sublethal UVA irradiation on activity levels of oxidative defense enzymes and protein oxidation in Escherichia coli

When bacterial cells are stressed by a change in the environment, they respond by changing the activity of enzymes at both the transcriptional and post-transcriptional levels. The UVA component (400-315 nm) of solar radiation reaching the Earth's surface is one of the most common stresses encountered by bacteria in their environment. Bacteria have evolved various antioxidant defense systems to increase survival when subjected to the deleterious effects of UVA irradiation. Recently, UVA-induced cytotoxicity and oxidative damage have been shown to be dependent on radiation intensity and dose distribution, not just total energy dose. We now report that when Escherichia coli is subjected to continuous sublethal, low-fluence UVA irradiation (7.4 W/m(2)) while growing to stationary phase, it responds by changing the activity levels of hydroperoxidases (HPI, HPII), glutathione reductase and manganese superoxide dismutase. This leads to an attenuation of the growth-delay response and an increase resistance to lethal UVA irradiation. When E. coli is given a UVA dose of 135 kJ/m(2) delivered at a fluence rate of 50 W/m(2), extensive protein oxidation occurs which may contribute to the inhibition of key cellular enzymes, leading to cellular dysfunction, DNA damage and eventually death. Changes in antioxidant enzymes induced by low-fluence UVA irradiation do not confer greater protection from protein oxidation after a challenge dose of UVA irradiation delivered at a fluence rate of 50 W/m(2).