Acetic acid induces pH-independent cellular energy depletion in Salmonella enterica

Evaluation of the Antibacterial Activity of Acetic Acid in Comparison With Three Disinfectants Against Bacteria Isolated From Hospital High-Touch Surfaces

Acetic acid, a readily available and less toxic alternative to conventional disinfectants, is widely used for cleaning in household settings. This study evaluates the antibacterial efficacy of acetic acid against bacteria isolated from hospital high-touch surfaces, comparing its performance to commonly used disinfectants, including phenol, sodium hypochlorite, and didecyldimethylammonium chloride (DDAC). A total of 120 samples were collected from high-touch surfaces in specialized patient areas. The antibacterial activity of acetic acid, phenol, sodium hypochlorite, and DDAC was assessed using the standard broth microdilution method against the isolated bacterial strains. From the 120 samples, 140 bacterial isolates were obtained. Acetic acid demonstrated strong antibacterial activity, with mean minimum inhibitory concentrations (MICs) ranging from 0.05 ± 0.00 to 0.25 ± 0.06 μL/mL, effectively inhibiting coagulase-negative Staphylococcus (CONS), Klebsiella pneumoniae, Proteus vulgaris, Enterococcus species, and Serratia marcescens. Its performance surpassed phenol and DDAC against these strains. Phenol exhibited higher MICs (0.50 ± 0.00 to 0.83 ± 0.10 μL/mL), indicating lower efficacy, while DDAC (0.06 ± 0.00 to 0.17 ± 0.04 μL/mL) and sodium hypochlorite (0.06 ± 0.00 to 0.10 ± 0.00 μL/mL) demonstrated comparable antibacterial effects. Phenol and sodium hypochlorite were found nonsignificant, while DDAC is highly effective at a concentration of 8.5%. Hospital surfaces were found to be contaminated with diverse bacterial strains. Acetic acid demonstrated significant antibacterial efficacy against both gram-positive and gram-negative bacteria, with MICs ranging from 0.05 ± 0.00 to 0.25 ± 0.06 μL/mL, suggesting its potential as an effective, economical, and less toxic alternative to conventional disinfectants.

Control of Escherichia coli O157:H7, Salmonella enterica serovar Typhimurium, and Listeria monocytogenes inoculated in beetroot or watermelon juice by combined treatments with organic acid or lemon (Citrus limon) extract and mild heat

The study aimed to evaluate the synergistic interaction of organic acids (OAAs) or lemon extract (LE) plus mild heat (MH; 55 °C) against Escherichia coli O157:H7, Salmonella enterica serovar Typhimurium, and Listeria monocytogenes inoculated in beetroot and watermelon juices. A mixed culture cocktail of E. coli O157:H7, S. Typhimurium or L. monocytogenes was inoculated in beetroot or watermelon juice, followed by treatments with MH, citric acid + MH, malic acid + MH, tartaric acid + MH, and LE + MH. Approximately < 2.0-log reductions in the number of E. coli O157:H7, S. Typhimurium, and L. monocytogenes were observed when these bacteria were heated in juices at 55 °C for 5 min. A combination of 1.0% OAAs or 20% LE and MH (55 °C) for 5 min resulted in an additional log-reduction in the count of E. coli O157:H7, S. Typhimurium, and L. monocytogenes by 2.2-5.0, 4.5-5.0, and 1.5-5.0, respectively.

Improving the alcohol respiratory chain and energy metabolism by enhancing PQQ synthesis in Acetobacter pasteurianus

Pyrroloquinoline quinone (PQQ) is one of the important coenzymes in living organisms. In acetic acid bacteria (AAB), it plays a crucial role in the alcohol respiratory chain, as a coenzyme of alcohol dehydrogenase (ADH). In this work, the PQQ biosynthetic genes were overexpressed in Acetobacter pasteurianus CGMCC 3089 to improve the fermentation performance. The result shows that the intracellular and extracellular PQQ contents in the recombinant strain A. pasteurianus (pBBR1-p264-pqq) were 152.53% and 141.08% higher than those of the control A. pasteurianus (pBBR1-p264), respectively. The catalytic activity of ADH and aldehyde dehydrogenase increased by 52.92% and 67.04%, respectively. The results indicated that the energy charge and intracellular ATP were also improved in the recombinant strain. The acetic acid fermentation was carried out using a 5 L self-aspirating fermenter, and the acetic acid production rate of the recombinant strain was 23.20% higher compared with the control. Furthermore, the relationship between the PQQ and acetic acid tolerance of cells was analyzed. The biomass of recombinant strain was 180.2%, 44.3%, and 38.6% higher than those of control under 2%, 3%, and 4% acetic acid stress, respectively. After being treated with 6% acetic acid for 40 min, the survival rate of the recombinant strain was increased by 76.20% compared with the control. Those results demonstrated that overexpression of PQQ biosynthetic genes increased the content of PQQ, therefore improving the acetic acid fermentation and the cell tolerance against acetic acid by improving the alcohol respiratory chain and energy metabolism.

ONE SENTENCE SUMMARY

The increase in PQQ content enhances the activity of the alcohol respiratory chain of Acetobacter pasteurianus, and the increase in energy charge enhances the tolerance of cells against acetic acid, therefore, improving the efficiency of acetic acid fermentation.

Anti-Salmonella mode of action of natural L-phenyl lactic acid purified from Lactobacillus plantarum ZJ316

Salmonella is a serious foodborne pathogen responsible for more than 90 million cases of gastroenteritis worldwide annually. Due to the gradual increase in antibiotic-resistant Salmonella strains, the identification of natural antibacterial substances is urgently needed. Herein, we purified natural L-phenyl lactic acid (L-PLA) from Lactobacillus plantarum ZJ316 and revealed its antimicrobial mode against Salmonella enterica subsp. enterica ATCC 14028. L-PLA (98.14% pure) was obtained using the macroporous resin XAD-16, solid-phase extraction (SPE), reverse-phase high-performance liquid chromatography (RP-HPLC), and chiral chromatography. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results showed that the cell morphology was severely compromised. Transmembrane electrical potential (ΔΨ), transmembrane pH gradient (ΔpH), intracellular ATP level, extracellular electrical conductivity (EC), and genomic DNA analyses were employed to evaluate the antibacterial mode of action of L-PLA. The proton motive force (PMF) and ATP of Salmonella cells rapidly dissipated, and the EC markedly increased. The gel retardation assay demonstrated that L-PLA could bind to genomic DNA and intercalate into the nucleic acids. The anti-Salmonella mode of action of L-PLA was attributed to the destruction of the cell membrane and genomic DNA binding. This research suggests that L-PLA has potential applications as an antimicrobial agent in food, medicine, and other fields. KEY POINTS: • Natural L-PLA was purified from L. plantarum ZJ316 with a purity of 98.14%. • L-PLA effectively inhibited Salmonella strains by antibacterial activities and MICs. • Membrane destruction and binding with DNA are the anti-Salmonella modes of L-PLA.

Salmonella enterica Filamentation Induced by Pelargonic Acid Is a Transient Morphotype

Under stressful conditions, Salmonella enterica forms multinucleated elongated filaments. The triggers and outcomes of filamentation are not well characterized. S. enterica serotypes Newport, Javiana, and Typhimurium were evaluated for their ability to form filaments upon exposure to 20 mM pelargonic acid. S. Newport was used as a model to investigate the progression and fate of filamentation via culturable population size, cell length, and viability assays. All serotypes displayed filament formation after 16 h of incubation. Pelargonic acid amendment of tryptic soy broth (TSBpel) produced a 5-log CFU reduction compared to TSB after 24 h (P < 0.05), and the growth rate decreased (P < 0.02). Cell elongation started within 12 h, peaked at 16 h, and was followed by filament disintegration at 20 to 24 h. The ratio of filaments to regular-sized cells (F/R) in TSBpel was 3.87 ± 0.59 at 16 h, decreasing to 0.23 ± 0.04 and 0.03 ± 0.01 (P < 0.05) at 20 and 24 h, respectively. Mg²⁺ supplementation repressed filamentation (F/R = 0.25 ± 0.11) and enhanced culturable cell counts (P < 0.05). Continued exposure to pelargonic acid inhibited growth in TSB and M9 compared to that in unamended media (P < 0.05). However, in M9 medium without Mg²⁺ amended with 20 mM pelargonic acid (M9pel), filament fragmentation progressed independently of pelargonic acid or Mg²⁺ When cells were pretreated with pelargonic acid to induce filamentation and then transferred to fresh medium, a positive effect of Mg²⁺ was noted under nutrient-deficient conditions, with higher live/dead cell ratios in M9 supplemented with 5 mM Mg²⁺ (M9Mg) than in M9 (P < 0.05). No change was observed when pelargonic acid was also added. Filamentation was ubiquitous in all serotypes tested, transient, and sensitive to Mg²⁺ Fragmentation, but not recovery, progressed irrespective of antimicrobial or Mg²⁺ presence.IMPORTANCE Some bacteria form elongated multinucleated structures, or filaments, when exposed to stress. The filamentous form of foodborne bacterial pathogens can interfere with food protection practices and diagnostic testing. Filamentation in Salmonella enterica Newport was investigated in response to pelargonic acid, a compound naturally found in several fruit and vegetables, and also used commercially as an herbicide. Salmonella readily formed filaments when exposed to pelargonic acid. Filaments were not stable, however, and fragmented to individual cells even when the fatty acid was still present, recovering fully when the stress was alleviated. A deeper exploration of the molecular mechanisms regulating filamentation and the conditions that induce it in agriculture and the food supply chain is needed to devise strategies that curb this response.

Disposition of Salmonella and Escherichia coli O157:H7 following Spraying of Contaminated Water on Cucumber Fruit and Flowers in the Field

Cucumbers are frequently consumed raw and have been implicated in several recent foodborne outbreaks. Because this item may become contaminated at the farm, it is vital to explore the fate of attenuated Salmonella Typhimurium or Escherichia coli O157:H7 sprayed onto foliage, flowers, and fruit in fields and determine whether pre- or postcontamination spray interventions could minimize contamination. After spraying cucumber plants with contaminated irrigation water (3.8 log CFU/mL of Salmonella Typhimurium and E. coli O157:H7), 60 to 78% of cucumber fruit were not contaminated because the plant's canopy likely prevented many of the underlying fruit from being exposed to the water. Subsequent exposure of contaminated cucumber plants to a simulated shower event did not appear to dislodge pathogens from contaminated foliage onto the fruit, nor did it appear to consistently wash either pathogen from the fruit. Spraying flowers and attached ovaries directly with a pathogen inoculum (4.6 log CFU/mL) initially led to 100% and 65 to 90% contamination, respectively. Within 3 days, 30 to 40% of the flowers were still contaminated; however, contamination of ovaries was minimal (≤10%), suggesting it was unlikely that internalization occurred through the flower to the ovary with these pathogen strains. In another study, both pathogens were found on a withered flower but not on the fruit to which the flower was attached, suggesting that this contaminated flower could serve as a source of cross-contamination in a storage bin if harvested with the fruit. Because pre- and postcontamination acetic acid-based spray treatments failed to reduce pathogen prevalence, the probability that fruit initially contaminated at 1.3 to 2.8 log CFU of Salmonella Typhimurium or E. coli O157:H7 per cucumber would be positive by enrichment culture decreased by a factor of 1.6 and 1.9 for Salmonella Typhimurium and E. coli O157:H7, respectively, for every day the fruit was held in the field ( P ≤ 0.0001). Hence, to reduce the prevalence of Salmonella Typhimurium on cucumbers below 5%, more than 1 week would be required.

Enhanced Antimicrobial Activity Based on a Synergistic Combination of Sublethal Levels of Stresses Induced by UV-A Light and Organic Acids

The reduction of microbial load in food and water systems is critical for their safety and shelf life. Conventionally, physical processes such as heat or light are used for the rapid inactivation of microbes, while natural compounds such as lactic acid may be used as preservatives after the initial physical process. This study demonstrates the enhanced and rapid inactivation of bacteria based on a synergistic combination of sublethal levels of stresses induced by UV-A light and two food-grade organic acids. A reduction of 4.7 ± 0.5 log CFU/ml in Escherichia coli O157:H7 was observed using a synergistic combination of UV-A light, gallic acid (GA), and lactic acid (LA), while the individual treatments and the combination of individual organic acids with UV-A light resulted in a reduction of less than 1 log CFU/ml. Enhanced inactivation of bacteria on the surfaces of lettuce and spinach leaves was also observed based on the synergistic combination. Mechanistic investigations suggested that the treatment with a synergistic combination of GA plus LA plus UV-A (GA+LA+UV-A) resulted in significant increases in membrane permeability and intracellular thiol oxidation and affected the metabolic machinery of E. coli In addition, the antimicrobial activity of the synergistic combination of GA+LA+UV-A was effective only against metabolically active E. coli O157:H7. In summary, this study illustrates the potential of simultaneously using a combination of sublethal concentrations of natural antimicrobials and a low level of physical stress in the form of UV-A light to inactivate bacteria in water and food systems.IMPORTANCE There is a critical unmet need to improve the microbial safety of the food supply, while retaining optimal nutritional and sensory properties of food. Furthermore, there is a need to develop novel technologies that can reduce the impact of food processing operations on energy and water resources. Conventionally, physical processes such as heat and light are used for inactivating microbes in food products, but these processes often significantly reduce the sensory and nutritional properties of food and are highly energy intensive. This study demonstrates that the combination of two natural food-grade antimicrobial agents with a sublethal level of physical stress in the form of UV-A light can greatly increase microbial load inactivation. In addition, this report elucidates the potential mechanisms for this synergistic interaction among physical and chemical stresses. Overall, these results provide a novel approach to develop antimicrobial solutions for food and water systems.