Dual-Species Biofilms Formed by Escherichia coli and Salmonella Enhance Chlorine Tolerance

Sanitization of hydroponic farming facilities in Singapore: what, why, and how

This study performed microbial analysis of nutrient film technique (NFT) hydroponic systems on three indoor farms in Singapore (the "what"). To justify the necessity of sanitizing hydroponic systems, strong biofilm-forming bacteria were isolated from the facility and investigated for their influence on Salmonella colonization on polyvinyl chloride (PVC) coupons in hydroponic nutrient solutions (the "why"). Finally, sanitization solutions were evaluated with both laboratory-scale and field-scale tests (the "how"). As a result, the microbiome composition in NFT systems was found to be highly farm specific. The strong biofilm formers Corynebacterium tuberculostearicum C2 and Pseudoxanthomonas mexicana C3 were found to facilitate the attachment and colonization of Salmonella on PVC coupons. When forming dual-species biofilms, the presence of C2 and C3 also significantly promoted the growth of Salmonella (P < 0.05). Compared with hydrogen peroxide (H2O2) and sodium percarbonate (SPC), sodium hypochlorite (NaOCl) exhibited superior efficacy in biofilm removal. At 50 ppm, NaOCl reduced the Salmonella Typhimurium, C2, and C3 counts to <1 log CFU/cm2 within 12 h, whereas neither 3% H2O2 nor 1% SPC achieved this effect. In operational hydroponic systems, the concentration of NaOCl needed to achieve biofilm elimination increased to 500 ppm, likely due to the presence of organic matter accumulated during crop cultivation and the greater persistence of naturally formed multispecies biofilms. Sanitization using 500 ppm NaOCl for 12 h did not impede subsequent plant growth, but chlorination byproduct chlorate was detected at high levels in the hydroponic solution and in plants in the sanitized systems without rinsing.

IMPORTANCE

This study's significance lies first in its elucidation of the necessity of sanitizing hydroponic farming systems. The microbiome in hydroponic systems, although mostly nonpathogenic, might serve as a hotbed for pathogen colonization and thus pose a risk for food safety. We thus explored sanitization solutions with both laboratory-scale and field-scale tests. Of the three tested sanitizers, NaOCl was the most effective and economical option, whereas one must note the vital importance of rinsing the hydroponic systems after sanitization with NaOCl.

Escherichia coli has an undiscovered ability to inhibit the growth of both Gram-negative and Gram-positive bacteria

The ability for bacteria to form boundaries between neighboring colonies as the result of intra-species inhibition has been described for a limited number of species. Here, we report that intra-species inhibition is more common than previously recognized. We demonstrated that swimming colonies of four Escherichia coli strains and six other bacteria form inhibitory zones between colonies, which is not caused by nutrient depletion. This phenomenon was similarly observed with non-flagellated bacteria. We developed a square-streaking pattern assay which revealed that Escherichia coli BW25113 inhibits the growth of other E. coli, and surprisingly, other Gram-positive and negative bacteria, including multi-drug resistant clinical isolates. Altogether, our findings demonstrate intra-species inhibition is common and might be used by E. coli to inhibit other bacteria. Our findings raise the possibility for a common mechanism shared across bacteria for intra-species inhibition. This can be further explored for a potential new class of antibiotics.

Glycogen plays a key role in survival of Salmonella Typhimurium on dry surfaces and in low-moisture foods

Salmonella enterica is known to survive in desiccate environments and is often associated with low-moisture foods (LMFs). In this work, S. Typhimurium ATCC 14028 was found to survive better by achieving the least reductions (3.17 ± 0.20 Log CFU reduction) compared to S. Tennessee ATCC 10722 (3.82 ± 0.13 Log CFU reduction) and S. Newport ATCC 6962 (6.03 ± 0.36 Log CFU reduction) after 30 days on surfaces with a relative humidity of 49% at ambient temperature. A metabolomic analysis revealed that S. Typhimurium was still active in energy metabolism after 24 h in the desiccate environment and glycogen, an energy reserve, was drastically reduced. We followed up on the glycogen levels over 30 days and found indeed a sharp decline on the first day. However, the glycogens detected on day 7 were significantly higher (P < 0.05) and thereafter remained stable above the original levels until day 30. The expression levels of both glycogen anabolism- and catabolism-related genes (csrA, glgA, glgC, glgX) were significantly up-regulated at all tested points (P < 0.05). The glgA and glgC insertion mutants displayed weaker survivability on both dry surfaces and in representative LMFs (flour and milk powder) compared to the wild-type strain. This work highlights the role of glycogen during different periods of desiccation, which may bring novel insight into mitigating Salmonella by disrupting glycogen metabolism.

Exposing Salmonella Senftenberg and Escherichia coli Strains Isolated from Poultry Farms to Formaldehyde and Lingonberry Extract at Low Concentrations

European Union (EU) countries strive to improve the quality and safety of food of animal origin. Food production depends on a good microbiological quality of fodder. However, feed can be a reservoir or vector of pathogenic microorganisms, including Salmonella or Escherichia coli bacteria. Salmonella spp. and E. coli are the two most important food-borne pathogens of public health concern. Contamination with these pathogens, mainly in the poultry sector, can lead to serious food-borne diseases. Both microorganisms can form biofilms on abiotic and biotic surfaces. The cells that form biofilms are less sensitive to disinfectants, which in turn makes it difficult to eliminate them from various surfaces. Because the usage of formaldehyde in animal feed is prohibited in European countries, the replacement of this antibacterial with natural plant products seems very promising. This study aimed to assess the inhibitory effectiveness of Vaccinium vitis-idaea extract against biofilm produced by model Salmonella enterica and E. coli strains. We found that formaldehyde could effectively kill both species of bacterial cells in biofilm, while the lingonberry extract showed some antibiofilm effect on S. enterica serovar Senftenberg. In conclusion, finding natural plant products that are effective against biofilms formed by Gram-negative bacteria is still challenging.