The effect of acid treatment on survival and protein expression of a laboratory K-12 strain Escherichia coli

Variability in the acid adaptation of ten different O157:H7 and non-O157 Escherichia coli strains in orange juice and the impact on UV radiation resistance

This study aimed to assess the impact of adaptation of ten strains of O157:H7 and non-O157 Escherichia coli to low pH (acid shock or slow acidification) and the effects of this exposure or not on the resistance of E. coli strains to UV radiation in orange juice (pH 3.5). The acid-shocked cells were obtained through culture in tryptic soy broth (TSB) with a final pH of 4.8, which was adjusted by hydrochloric, lactic, or citric acid and subsequently inoculated in orange juice at 4 °C for 30 days. No significant differences (p > 0.05) in survival in orange juice were observed between the serotypes O157:H7 and non-O157:H7 for acid-shocked experiments. After slow acidification, where the cells were cultured in TSB supplemented with glucose 1% (TSB + G), a significant increase (p < 0.05) in survival was observed for all strains evaluated. The D-values (radiation dose (J/cm2) necessary to decrease the microbial population by 90%) were determined as the inverse of the slopes of the regressions (k) obtained by plotting log (N/N0). The results show that among the strains tested, E. coli O157:H7 (303/00) and O26:H11 were the most resistant and sensitive strains, respectively. According to our results, the method of acid adaptation contributes to increasing the UV resistance for most of the strains tested.

Microbial production of mevalonate by recombinant Escherichia coli using acetic acid as a carbon source

We sought to produce mevalonate, an important organic acid, by recombinant Escherichia coli, using acetic acid, a less costly alternative feedstock, as a carbon source. In this study, the mevalonate biosynthesis pathway originating with acetate was constructed in recombinant E. coli, resulting in the production of 1.06 g L^-1 mevalonate with a productivity of 0.03 g L^-1 h^-1 in a 5-L bioreactor. The mevalonate concentration and productivity were significantly enhanced with increased cell density during 2-stage aerobic fermentation, reaching 7.85 g L^-1 and 0.13 g L^-1 h^-1, respectively. Fed-batch fermentation was further optimized under anaerobic and microaerobic conditions, and mevalonate concentrations reached 3.05 g L^-1 and 4.97 g L^-1, respectively, indicating that the oxygen supply exerts a large impact on mevalonate production from acetate. This study describes a method with high potential to produce mevalonate with the engineered E. coli strain XU143 using the less costly alternative feedstock acetate as a carbon source.

An Introduction to the Avian Gut Microbiota and the Effects of Yeast-Based Prebiotic-Type Compounds as Potential Feed Additives

The poultry industry has been searching for a replacement for antibiotic growth promoters in poultry feed as public concerns over the use of antibiotics and the appearance of antibiotic resistance has become more intense. An ideal replacement would be feed amendments that could eliminate pathogens and disease while retaining economic value via improvements on body weight and feed conversion ratios. Establishing a healthy gut microbiota can have a positive impact on growth and development of both body weight and the immune system of poultry while reducing pathogen invasion and disease. The addition of prebiotics to poultry feed represents one such recognized way to establish a healthy gut microbiota. Prebiotics are feed additives, mainly in the form of specific types of carbohydrates that are indigestible to the host while serving as substrates to select beneficial bacteria and altering the gut microbiota. Beneficial bacteria in the ceca easily ferment commonly studied prebiotics, producing short-chain fatty acids, while pathogenic bacteria and the host are unable to digest their molecular bonds. Prebiotic-like substances are less commonly studied, but show promise in their effects on the prevention of pathogen colonization, improvements on the immune system, and host growth. Inclusion of yeast and yeast derivatives as probiotic and prebiotic-like substances, respectively, in animal feed has demonstrated positive associations with growth performance and modification of gut morphology. This review will aim to link together how such prebiotics and prebiotic-like substances function to influence the native and beneficial microorganisms that result in a diverse and well-developed gut microbiota.

Host origin determines pH tolerance of Tritrichomonas foetus isolates from the feline gastrointestinal and bovine urogenital tracts

The ability for protozoan parasites to tolerate pH fluctuations within their niche is critical for the establishment of infection and require the parasite to be capable of adapting to a distinct pH range. We used two host adapted Tritrichomonas foetus isolates, capable of infecting either the digestive tract (pH 5.3-6.6) of feline hosts or the reproductive tract (pH 7.4-7.8) of bovine hosts to address their adaptability to changing pH. Using flow cytometry, we investigated the pH tolerance of the bovine and feline T. foetus isolates over a range of physiologically relevant pH in vitro. Following exposure to mild acid stress (pH 6), the bovine T. foetus isolates showed a significant decrease in cell viability and increased cytoplasmic granularity (p-value < 0.003, p-value < 0.0002) compared to pH 7 and 8 (p-value > 0.7). In contrast, the feline genotype displayed an enhanced capacity to maintain cell morphology and viability (p-value > 0.05). Microscopic assessment revealed that following exposure to a weak acidic stress (pH 6), the bovine T. foetus transformed into rounded parasites with extended cell volumes and displays a decrease in viability. The higher tolerance for acidic extracellular environment of the feline isolate compared to the bovine isolate suggests that pH could be a critical factor in regulating T. foetus infections and host-specificity.

Control of the Biofilms Formed by Curli- and Cellulose-Expressing Shiga Toxin-Producing Escherichia coli Using Treatments with Organic Acids and Commercial Sanitizers

Biofilms are a mixture of bacteria and extracellular products secreted by bacterial cells and are of great concern to the food industry because they offer physical, mechanical, and biological protection to bacterial cells. This study was conducted to quantify biofilms formed by different Shiga toxin-producing Escherichia coli (STEC) strains on polystyrene and stainless steel surfaces and to determine the effectiveness of sanitizing treatments in control of these biofilms. STEC producing various amounts of cellulose (n = 6) or curli (n = 6) were allowed to develop biofilms on polystyrene and stainless steel surfaces at 28°C for 7 days. The biofilms were treated with 2% acetic or lactic acid and manufacturer-recommended concentrations of acidic or alkaline sanitizers, and residual biofilms were quantified. Treatments with the acidic and alkaline sanitizers were more effective than those with the organic acids for removing the biofilms. Compared with their counterparts, cells expressing a greater amount of cellulose or curli formed more biofilm mass and had greater residual mass after sanitizing treatments on polystyrene than on stainless steel. Research suggests that the organic acids and sanitizers used in the present study differed in their ability to control biofilms. Bacterial surface components and cell contact surfaces can influence both biofilm formation and the efficacy of sanitizing treatments. These results provide additional information on control of biofilms formed by STEC.

Acid response of exponentially growing Escherichia coli K-12

Induction of acid tolerance response (ATR) of exponential-phase Escherichia coli K-12 cells grown and adapted at different conditions was examined. The highest level of protection against pH 2.5 challenges was obtained after adaptation at pH 4.5-4.9 for 60 min. To study the genetic systems, which could be involved in the development of log-phase ATR, we investigated the acid response of E. coli acid resistance (AR) mutants. The activity of the glutamate-dependent system was observed in exponential cells grown at pH 7.0 and acid adapted at pH 4.5 in minimal medium. Importantly, log-phase cells exhibited significant AR when grown in minimal medium pH 7.0 and challenged at pH 2.5 for 2 h without adaptation. This AR required the glutamate-dependent AR system. Acid protection was largely dependent on RpoS in unadapted and adapted cells grown in minimal medium. RpoS-dependent oxidative, glutamate and arginine-dependent decarboxylase AR systems were not involved in triggering log-phase ATR in cells grown in rich medium. Cells adapted at pH 4.5 in rich medium showed a higher proton accumulation rate than unadapted cells as determined by proton flux assay. It is clear from our study that highly efficient mechanisms of protection are induced, operate and play the main role during log-phase ATR.

Effects of acid stress on Vibrio parahaemolyticus survival and cytotoxicity

For several foodborne bacterial pathogens, an acid tolerance response appears to be an important strategy for counteracting acid stress imposed either during food processing or by the human host. The acid tolerance response enhances bacterial survival of lethal acid challenge following prior exposure to sublethal acidic conditions. Previous studies have revealed relationships between a foodborne pathogen's ability to survive acid challenge and its infectious dose. Vibrio parahaemolyticus is capable of causing gastroenteritis when sufficient cells of pathogenic strains are consumed. This study was designed to characterize acid sensitivities and to compare the effects of sublethal acid exposure (adaptation) on survival capabilities and cytotoxicities of different V. parahaemolyticus strains. Survival of acid challenge by stationary-phase cells differed by up to 3 log CFU/ml among the 25 isolates tested. No differences in acid resistance were found between strains when they were grouped by source (clinical isolates versus those obtained from food). Survival at pH 3.6 for log-phase cells that had been previously exposed to sublethal acidic conditions (pH 5.5) was enhanced compared with that for cells not previously exposed to pH 5.5. However, for stationary-phase cells, exposure to pH 5.5 impaired both subsequent survival at pH 3.6 and cytotoxicity to human epithelial cells. Relative cytotoxicities of nonadapted stationary-phase cells were 1.2- to 4.8-fold higher than those of adapted cells. Sublethal acid exposure appears to impose measurable growth phase-dependent effects on subsequent lethal acid challenge survival and cytotoxicity of V. parahaemolyticus.