Genetic determinants of increased sodium hypochlorite and ciprofloxacin susceptibility in Pseudomonas aeruginosa PA14 biofilms

Reactive chlorine species (RCS) like sodium hypochlorite (NaOCl) are potent oxidizing agents and widely used biocides in surface disinfection, water treatment, and biofilm elimination. Moreover, RCS are also produced by the human immune system to kill invading pathogens. However, bacteria have developed mechanisms to survive the damage caused by RCS. Using the comprehensive Pseudomonas aeruginosa PA14 transposon mutant library in a genetic screen, we identified a total of 28 P. aeruginosa PA14 mutants whose biofilms showed increased susceptibility to NaOCl in comparison to PA14 WT biofilms. Of these, ten PA14 mutants with a disrupted apaH, PA0793, acsA, PA1506, PA1547, PA3728, yajC, queA, PA3869, or PA14_32840 gene presented a 4-fold increase in NaOCl susceptibility compared to wild-type biofilms. While none of these mutants showed a defect in biofilm formation or attenuated susceptibility of biofilms toward the oxidant hydrogen peroxide (H2O2), all but PA14_32840 also exhibited a 2-4-fold increase in susceptibility toward the antibiotic ciprofloxacin. Further analyses revealed attenuated levels of intracellular ROS and catalase activity only for the apaH and PA1547 mutant, providing insights into the oxidative stress response in P. aeruginosa biofilms. The findings of this paper highlight the complexity of biofilm resistance and the intricate interplay between different mechanisms to survive oxidative stress. Understanding resistance strategies adopted by biofilms is crucial for developing more effective ways to fight resistant bacteria, ultimately contributing to better management of bacterial growth and resistance in clinical and environmental settings.

Oxidative stress responses in biofilms

Oxidizing agents are low-molecular-weight molecules that oxidize other substances by accepting electrons from them. They include reactive oxygen species (ROS), such as superoxide anions (O2-), hydrogen peroxide (H2O2), and hydroxyl radicals (HO-), and reactive chlorine species (RCS) including sodium hypochlorite (NaOCl) and its active ingredient hypochlorous acid (HOCl), and chloramines. Bacteria encounter oxidizing agents in many different environments and from diverse sources. Among them, they can be produced endogenously by aerobic respiration or exogenously by the use of disinfectants and cleaning agents, as well as by the mammalian immune system. Furthermore, human activities like industrial effluent pollution, agricultural runoff, and environmental activities like volcanic eruptions and photosynthesis are also sources of oxidants. Despite their antimicrobial effects, bacteria have developed many mechanisms to resist the damage caused by these toxic molecules. Previous research has demonstrated that growing as a biofilm particularly enhances bacterial survival against oxidizing agents. This review aims to summarize the current knowledge on the resistance mechanisms employed by bacterial biofilms against ROS and RCS, focussing on the most important mechanisms, including the formation of biofilms in response to oxidative stressors, the biofilm matrix as a protective barrier, the importance of detoxifying enzymes, and increased protection within multi-species biofilm communities. Understanding the complexity of bacterial responses against oxidative stress will provide valuable insights for potential therapeutic interventions and biofilm control strategies in diverse bacterial species.

Antimicrobial Tolerance in Salmonella: Contributions to Survival and Persistence in Processing Environments

Salmonella remains a top bacterial pathogen implicated in several food-borne outbreaks, despite the use of antimicrobials and sanitizers during production and processing. While these chemicals have been effective, Salmonella has shown the ability to survive and persist in poultry processing environments. This can be credited to its microbial ability to adapt and develop/acquire tolerance and/or resistance to different antimicrobial agents including oxidizers, acids (organic and inorganic), phenols, and surfactants. Moreover, there are several factors in processing environments that can limit the efficacy of these antimicrobials, thus allowing survival and persistence. This mini-review examines the antimicrobial activity of common disinfectants/sanitizers used in poultry processing environments and the ability of Salmonella to respond with innate or acquired tolerance and survive exposure to persists in such environments. Instead of relying on a single antimicrobial agent, the right combination of different disinfectants needs to be developed to target multiple pathways within Salmonella.

Biofilm formation in food processing plants and novel control strategies to combat resistant biofilms: the case of Salmonella spp

Salmonella is one of the pathogens that cause many foodborne outbreaks throughout the world, representing an important global public health problem. Salmonella strains with biofilm-forming abilities have been frequently isolated from different food processing plants, especially in poultry industry. Biofilm formation of Salmonella on various surfaces can increase their viability, contributing to their persistence in food processing environments and cross-contamination of food products. In recent years, increasing concerns arise about the antimicrobial resistant and disinfectant tolerant Salmonella, while adaptation of Salmonella in biofilms to disinfectants exacerbate this problem. Facing difficulties to inhibit or remove Salmonella biofilms in food industry, eco-friendly and effective strategies based on chemical, biotechnological and physical methods are in urgent need. This review discusses biofilm formation of Salmonella in food industries, with emphasis on the current available knowledge related to antimicrobial resistance, together with an overview of promising antibiofilm strategies for controlling Salmonella in food production environments.

Sublethal chlorine stress promotes the biofilm-forming ability of Salmonella enterica serovars enteritidis and expression of the related genes

Chlorine treatment is the most common disinfection method in food-related environments. In addition to being simple and inexpensive, this method is very effective if used properly. However, insufficient chlorine concentrations only cause a sublethal oxidative stress in the bacterial population and may alter the growth behavior of stressed cells. In the present study, the effect of sublethal chlorine stress on the biofilm formation characteristics of Salmonella Enteritidis was evaluated. Our results demonstrated that, sublethal chlorine stress (350 ppm total chlorine) activates the biofilm (csgD, agfA, adrA and bapA) and quorum-sensing (sdiA and luxS) related genes in planktonic cells of S. Enteritidis. The higher expression of these genes illustrated that the chlorine stress induced the initiation of the biofilm formation process in S. Enteritidis. Results of the initial attachment assay confirmed this finding. In addition, the number of chlorine-stressed biofilm cells was significantly higher than non-stressed biofilm cells after 48 h incubation at 37 °C. In S. Enteritidis ATCC 13076 and S. Enteritidis KL19, the number of chlorine-stressed biofilm cells were 6.93 ± 0.48 and 7.49 ± 0.57 log CFU/cm², while the number of non-stressed biofilm cells were 5.12 ± 0.39 and 5.63 ± 0.51 log CFU/cm², respectively. These findings were confirmed by measurements of the major components of biofilm, i.e., eDNA, protein and carbohydrate. The amount of these components in 48-h biofilms was higher when the cells were initially subjected to sublethal chlorine stress. However, the up-regulation of the biofilm and quorum sensing genes was not observed in 48-h biofilm cells, indicating that the effect of chlorine stress had vanished in the subsequent generations of Salmonella. In total, these results revealed that sublethal chlorine concentrations can promote the biofilm-forming ability of S Enteritidis.

Ethanol at Subinhibitory Concentrations Enhances Biofilm Formation in Salmonella Enteritidis

The survival of Salmonella Enteritidis in the food chain is relevant to its biofilm formation capacity, which is influenced by suboptimal environmental conditions. Here, biofilm formation pattern of this bacterium was assessed in the presence of ethanol at sub-minimal inhibitory concentrations (sub-MICs) by microtiter plate assays, cell characteristic analyses, and gene expression tests. It was observed that ethanol at subinhibitory concentrations (1/4 MIC, 2.5%; 1/2 MIC, 5.0%) was able to stimulate biofilm formation in S. Enteritidis. The OD₅₉₅ value (optical density at 595 nm) used to quantify biofilm production was increased from 0.14 in control groups to 0.36 and 0.63 under 2.5% and 5.0% ethanol stresses, respectively. Ethanol was also shown to reduce bacterial swimming motility and enhance cell auto-aggregation ability. However, other cell characteristics such as swarming activity, initial attachment and cell surface hydrophobicity were not remarkedly impacted by ethanol. Reverse transcription quantitative real-time PCR (RT-qPCR) analysis further revealed that the luxS gene belonging to a quorum-sensing system was upregulated by 2.49- and 10.08-fold in the presence of 2.5% and 5.0% ethanol, respectively. The relative expression level of other biofilm-related genes (adrA, csgB, csgD, and sdiA) and sRNAs (ArcZ, CsrB, OxyS, and SroC) did not obviously change. Taken together, these findings suggest that decrease in swimming motility and increase in cell auto-aggregation and quorum sensing may result in the enhancement of biofilm formation by S. Enteritidis under sublethal ethanol stress.

An Evaluation of the Pathogenic Potential, and the Antimicrobial Resistance, of Salmonella Strains Isolated from Mussels

Salmonella spp. and antimicrobial resistant microorganisms are two of the most important health issues worldwide. In the present study, strains naturally isolated from mussels harvested in Galicia (one of the main production areas in the world), were genetically characterized attending to the presence of virulence and antimicrobial resistance genes. Additionally, the antimicrobial profile was also determined phenotypically. Strains presenting several virulence genes were isolated but lacked all the antimicrobial resistance genes analyzed. The fact that some of these strains presented multidrug resistance, highlighted the possibility of bearing different genes than those analyzed, or resistance based on completely different mechanisms. The current study highlights the importance of constant surveillance in order to improve the safety of foods.

Differences in biofilm formation of Salmonella serovars on two surfaces under two temperature conditions

AIMS

Salmonella is extremely diverse, with >2500 serovars that are genetically and phenotypically diverse. The aim of this study was to build a collection of Salmonella isolates that are genetically diverse and to evaluate their ability to form biofilm under different conditions relevant to a processing environment.

METHODS AND RESULTS

Twenty Salmonella isolates representative of 10 serovars were subtyped using Clustered regularly interspaced short palindromic repeats (CRISPR)-typing to assess the genetic diversity between isolates of each serovar. Biofilm formation of the isolates on both plastic and stainless-steel surfaces at 25 and 15°C was assessed. At 25°C, 8/20 isolates each produced strong and moderate biofilm on plastic surface compared to stainless-steel (3/20 and 13/20 respectively). At 15°C, 5/20 produced strong biofilm on plastic surface and none on stainless-steel. Several isolates produced weak biofilm on plastic (11/20) and stainless-steel (16/20) surfaces. Serovar Schwarzengrund consistently produced strong biofilm while serovars Heidelberg and Newport produced weak biofilm.

CONCLUSION

These results suggest that Salmonellae differ in their attachment depending on the surface and temperature conditions encountered, which may influence persistence in the processing environment.

SIGNIFICANCE AND IMPACT OF STUDY

These differences in biofilm formation could provide useful information for mitigation of Salmonella in processing environments.

Antimicrobials and Food-Related Stresses as Selective Factors for Antibiotic Resistance along the Farm to Fork Continuum

Antimicrobial resistance (AMR) is a global problem and there has been growing concern associated with its widespread along the animal-human-environment interface. The farm-to-fork continuum was highlighted as a possible reservoir of AMR, and a hotspot for the emergence and spread of AMR. However, the extent of the role of non-antibiotic antimicrobials and other food-related stresses as selective factors is still in need of clarification. This review addresses the use of non-antibiotic stressors, such as antimicrobials, food-processing treatments, or even novel approaches to ensure food safety, as potential drivers for resistance to clinically relevant antibiotics. The co-selection and cross-adaptation events are covered, which may induce a decreased susceptibility of foodborne bacteria to antibiotics. Although the available studies address the complexity involved in these phenomena, further studies are needed to help better understand the real risk of using food-chain-related stressors, and possibly to allow the establishment of early warnings of potential resistance mechanisms.

A novel concentration gradient microfluidic chip for high-throughput antibiotic susceptibility testing of bacteria

Antibiotic resistance has become a serious threat to food safety and public health globally. Therefore, the development of a sensitive, quick, and simple method for antibiotic susceptibility testing is an urgent and crucial need. A novel concentration gradient microfluidic chip was designed in this work to generate antibiotic concentration gradient, culture bacteria, and produce fluorescence emission. An in-house-assembled fluorescence detection platform was constructed, and experiments were conducted to verify the linearity of the generated concentration gradient, explore the appropriate incubation time and flow rate for the microfluidic chip, and study the effect of long-term acid-based food processing on antibiotic susceptibility testing. Experimental results show that the concentration gradient generated by the microfluidic chip exhibited good linearity, stability, and controllability. The appropriate flow rate and incubation time for the microfluidic chip were 2 μL/min and 5 h, respectively. The use of this microfluidic chip for testing antibiotic resistance of Salmonella to ofloxacin and ampicillin generated results that were completely consistent with test results obtained using the gold-standard method. Furthermore, Salmonella showed greater sensitivity to antibiotics under strong acid conditions, confirming the potential influence of acid-based food processing on antibiotic susceptibility testing of real samples. The designed microfluidic chip provides a high-throughput, sensitive, and rapid antibiotic susceptibility testing method that combines the microfluidic chip and the fluorescence detection platform. The application of this method would facilitate determination of antibiotic-resistant bacterial strains for clinicians and researchers, and enable monitoring of changes in bacterial resistance during food processing.

Surviving Reactive Chlorine Stress: Responses of Gram-Negative Bacteria to Hypochlorous Acid

Sodium hypochlorite (NaOCl) and its active ingredient, hypochlorous acid (HOCl), are the most commonly used chlorine-based disinfectants. HOCl is a fast-acting and potent antimicrobial agent that interacts with several biomolecules, such as sulfur-containing amino acids, lipids, nucleic acids, and membrane components, causing severe cellular damage. It is also produced by the immune system as a first-line of defense against invading pathogens. In this review, we summarize the adaptive responses of Gram-negative bacteria to HOCl-induced stress and highlight the role of chaperone holdases (Hsp33, RidA, Cnox, and polyP) as an immediate response to HOCl stress. We also describe the three identified transcriptional regulators (HypT, RclR, and NemR) that specifically respond to HOCl. Besides the activation of chaperones and transcriptional regulators, the formation of biofilms has been described as an important adaptive response to several stressors, including HOCl. Although the knowledge on the molecular mechanisms involved in HOCl biofilm stimulation is limited, studies have shown that HOCl induces the formation of biofilms by causing conformational changes in membrane properties, overproducing the extracellular polymeric substance (EPS) matrix, and increasing the intracellular concentration of cyclic-di-GMP. In addition, acquisition and expression of antibiotic resistance genes, secretion of virulence factors and induction of the viable but nonculturable (VBNC) state has also been described as an adaptive response to HOCl. In general, the knowledge of how bacteria respond to HOCl stress has increased over time; however, the molecular mechanisms involved in this stress response is still in its infancy. A better understanding of these mechanisms could help understand host-pathogen interactions and target specific genes and molecules to control bacterial spread and colonization.

Antimicrobial resistance associated with the use of antimicrobial processing aids during poultry processing operations: cause for concern?

Antimicrobial resistance has become a global issue and a threat to human and animal health. Contamination of poultry carcasses with meat-borne pathogens represents both an economic and a public health concern. The use of antimicrobial processing aids (APA) during poultry processing has contributed to an improvement in the microbiological quality of poultry carcasses. However, the extensive use of these decontaminants has raised concerns about their possible role in the co-selection of antibiotic-resistant bacteria. This topic is presented in the current review to provide an update on the information related to bacterial adaptation to APA used in poultry processing establishments, and to discuss the relationship between APA bacterial adaptation and the acquisition of a new resistance phenotype to therapeutic antimicrobials by bacteria. Common mechanisms such as active efflux and changes in membrane fluidity are the most documented mechanisms responsible for bacterial cross-resistance to APA and antimicrobials. Although most studies reported a bacterial resistance to antibiotics not reaching a clinical level, the under-exposure of bacteria to APA remains a concern in the poultry industry. Further research is needed to determine if APA used during poultry processing and therapeutic antimicrobials share common sites of action in bacteria and encounter similar mechanisms of resistance.

Borderline resistance to oxacillin in Staphylococcus aureus after treatment with sub-lethal sodium hypochlorite concentrations

Surface disinfectants are regularly used in prophylactic and infection control measures. Concern has been raised whether residues of sub-inhibitory disinfectant concentrations may constitute a selective pressure and could contribute to the development of strains which are tolerant and/or resistant to biocides including antibiotics. The current study investigated whether Staphylococcus (S.) aureus ATCC® 29213™ and ATCC® 6538™ would change their growth characteristics and antimicrobial susceptibility profiles after prolonged treatment with sub-inhibitory concentrations of sodium hypochlorite (NaOCl). NaOCl is a fast-acting disinfectant with a broad-spectrum activity, inexpensive and widely used in healthcare and the food production industry. Minimum inhibitory concentration (MIC) for NaOCl was determined by broth macrodilution according to the guidelines for disinfectant efficacy testing provided by the German Veterinary Medical Society. Serial passages after 24 h and 72 h, respectively, in defined sub-inhibitory concentrations of NaOCl resulted in a number of phenotypic variants. Two of these variants, derived from S. aureus ATCC® 29213™, showed elevated MICs of oxacillin and were considered as in vitro-generated borderline oxacillin-resistant S. aureus (BORSA). Transmission electron microscopy revealed a significantly thickened cell wall in these isolates, a phenomenon that has also been described for Listeria monocytogenes after low-level exposure to NaOCl. Whole genome sequencing revealed an early stop codon in the gene coding for the GdpP protein and thereby abolishing the function of this gene. GdpP represents a phosphodiesterase that regulates gene expression, and loss of function of the GdpP protein has been described in association with borderline oxacillin resistance. Our findings suggest that a mutation in the GdpP protein gene and morphological changes of the cell wall were induced by repeated exposure to sub-lethal NaOCl concentrations, and most likely accounted for a BORSA phenotype in two variants derived from S. aureus ATCC® 29213™.

Rugose Morphotype in Salmonella Typhimurium and Salmonella Heidelberg Induced by Sequential Exposure to Subinhibitory Sodium Hypochlorite Aids in Biofilm Tolerance to Lethal Sodium Hypochlorite on Polystyrene and Stainless Steel Surfaces

Salmonella biofilms act as a continuous source for cross-contamination in the food processing environments. In this study, a stable rugose morphotype of Salmonella was first induced by sequential exposure to subinhibitory concentrations (SICs) of sodium hypochlorite (NaOCl) (ranging from 50 to 300 ppm over 18-day period) in tryptic soy broth. Then, rugose and smooth morphotypes of Salmonella Typhimurium ATCC 14028 and Salmonella Heidelberg ATCC 8326 were characterized for biofilm forming abilities on polystyrene and stainless steel surfaces. Rugose morphotype of both ATCC 14028 and ATCC 8326 exhibited higher Exopolysaccharide (EPS) formation than smooth morphotype (p ≤ 0.05). Also, the SICs of NaOCl (200 or 300 ppm in broth model) increased the biofilm formation ability of rugose morphotype of ATCC 8326 (p ≤ 0.05) but decreased that of ATCC 14028. The 2-day-old Salmonella biofilms were treated with biocidal concentrations of 50, 100, or 200 ppm NaOCl (pH 6.15) in water for 5, 10, or 20 min at room temperature. The biofilm reduction in CFU/cm2 for the rugose was lower than the smooth morphotype on both surfaces (p ≤ 0.05) by lethal NaOCl in water. Scanning electron micrographs on both polystyrene and stainless steel surfaces demonstrated that the rugose morphotype produced a denser biofilm than the smooth morphotype. Transmission electron micrographs revealed the cell wall roughness in rugose morphotype, which may help in tolerance to NaOCl. The gene expression data indicate that the expression of biofilm regulator (csgD), curli (csgA, csgB, and csgC), and cellulose (bcsE) was significantly increased in rugose morphotype when induced by sequential exposure of NaOCl SICs. These findings reveal that the rugose morphotype of S. Typhimurium and S. Heidelberg produced significantly denser biofilm on food contact surfaces, which also increased with sequential exposure to SICs of NaOCl in the case of S. Heidelberg, and these biofilms were more tolerant to biocidal NaOCl concentrations commonly used in the food processing plants.

Exploitation of plant extracts and phytochemicals against resistant Salmonella spp. in biofilms

Salmonella is one of the most frequent causes of foodborne outbreaks throughout the world. In the last years, the resistance of this and other pathogenic bacteria to antimicrobials has become a prime concern towards their successful control. In addition, the tolerance and virulence of pathogenic bacteria, such as Salmonella, are commonly related to their ability to form biofilms, which are sessile structures encountered on various surfaces and whose development is considered as a universal stress response mechanism. Indeed, the ability of Salmonella to form a biofilm seems to significantly contribute to its persistence in food production areas and clinical settings. Plant extracts and phytochemicals appear as promising sources of novel antimicrobials due to their cost-effectiveness, eco-friendliness, great structural diversity, and lower possibility of antimicrobial resistance development in comparison to synthetic chemicals. Research on these agents mainly attributes their antimicrobial activity to a diverse array of secondary metabolites. Bacterial cells are usually killed by the rupture of their cell envelope and in parallel the disruption of their energy metabolism when treated with such molecules, while their use at sub-inhibitory concentrations may also disrupt intracellular communication. The purpose of this article is to review the current available knowledge related to antimicrobial resistance of Salmonella in biofilms, together with the antibiofilm properties of plant extracts and phytochemicals against these detrimental bacteria towards their future application to control these in food production and clinical environments.