Phenotypic susceptibility testing of Escherichia (E.) coli is an essential tool to gain a better understanding of the potential impact of biocide selection pressure on antimicrobial resistance. We, therefore, determined the biocide and antimicrobial susceptibility of 216 extended-spectrum β-lactamase-producing (ESBL) and 177 non-ESBL E. coli isolated from swine feces, pork meat, voluntary donors and inpatients and evaluated associations between their susceptibilities. Minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) of benzalkonium chloride, chlorhexidine digluconate (CHG), chlorocresol (PCMC), glutaraldehyde (GDA), isopropanol (IPA), octenidine dihydrochloride and sodium hypochlorite (NaOCl) showed unimodal distributions, indicating the absence of bacterial adaptation to biocides due to the acquisition of resistance mechanisms. Although MIC₉₅ and MBC₉₅ did not vary more than one doubling dilution step between isolates of porcine and human origin, significant differences in MIC and/or MBC distributions were identified for GDA, CHG, IPA, PCMC and NaOCl. Comparing non-ESBL and ESBL E. coli, significantly different MIC and/or MBC distributions were found for PCMC, CHG and GDA. Antimicrobial susceptibility testing revealed the highest frequency of resistant E. coli in the subpopulation isolated from inpatients. We observed significant but weakly positive correlations between biocide MICs and/or MBCs and antimicrobial MICs. In summary, our data indicate a rather moderate effect of biocide use on the susceptibility of E. coli to biocides and antimicrobials.

Disinfection is a key strategy to reduce the burden of infections. The contact of bacteria to biocides—the active substances of disinfectants—has been linked to bacterial adaptation and the development of antimicrobial resistance. Currently, there is no scientific consensus on whether the excessive use of biocides contributes to the emergence and spread of multidrug resistant bacteria. The comprehensive analysis of available data remains a challenge because neither uniform test procedures nor standardized interpretive criteria nor harmonized terms are available to describe altered bacterial susceptibility to biocides. In our review, we investigated the variety of criteria and the diversity of terms applied to interpret findings in original studies performing biocide susceptibility testing (BST) of field isolates. An additional analysis of reviews summarizing the knowledge of individual studies on altered biocide susceptibility provided insights into currently available broader concepts for data interpretation. Both approaches pointed out the urgent need for standardization. We, therefore, propose that the well-established and approved concepts for interpretation of antimicrobial susceptibility testing data should serve as a role model to evaluate biocide resistance mechanisms on a single cell level. Furthermore, we emphasize the adaptations necessary to acknowledge the specific needs for the evaluation of BST data. Our approach might help to increase scientific awareness and acceptance.

Inadequate hygiene measures as well as the use of contaminated inks or non-sterile needles are considered as important infection sources in the process of tattooing. In tattoo parlors and at conventions, it is common practice to apply cosmetic products from bulk packs as lubricants during tattooing and as ointments for tattoo aftercare. The objective of our study was to assess the microbial load of opened skin care products used during tattooing or for tattoo aftercare. First, we established a homogenization method suitable for microbiological examination of water immiscible products. To this end, we compared the efficiency of FastPrep^TM and Stomacher® homogenizers on artificially contaminated petroleum jelly. FastPrep homogenates revealed significantly higher detection rates (≥ 97%) compared to Stomacher ones (31-64%). Second, we investigated 106 cosmetic bulk pack products collected from tattoo artists. After FastPrep homogenization for 30 seconds total aerobic mesophilic bacteria and the presence of Pseudomonas aeruginosa, Staphylococcus aureus and Candida albicans were determined through culture. We also tested for Mycobacteria spp. by qPCR. In total, 4.7% of the cosmetic products under study turned out to be contaminated. The observed microbial contamination of opened skin care bulk packs can hold a risk to introduce bacteria into the fresh skin wound resulting from tattooing and may be a risk factor for post-tattoo infections.

Foodborne pathogens pose a persistent risk to public health, with domestic environments representing a major but often underestimated source of contamination. In this study, we investigated the survival, proliferation, and transfer potential of Salmonella Enteritidis, Escherichia coli, and Staphylococcus aureus in kitchen sponges harboring an established core microbiota. Using culture-based, metagenomic, and fluorescence in situ hybridization approaches in combination with confocal laser scanning microscopy, we examined pathogen persistence, desiccation tolerance, cross-contamination potential, and spatial microbial organization over 14 days. All three pathogens persisted within the sponge matrix for at least 2 weeks, even at very low initial populations (approximately 2,5 log₁₀ colony-forming units (CFU) per sponge section). Escherichia coli and Salmonella Enteritidis rapidly established stable populations reaching approximately 9 log CFU per sponge section, whereas S. aureus showed limited growth of approximately 4 log CFU per sponge section, indicating species-specific interactions with the resident microbiota. Notably, pathogen populations remained stable after 3 days of desiccation, confirming the role of sponges as long-term microbial reservoirs. Contact between colonized sponges and surfaces under mild pressure resulted in transfer of up to 5 log CFU to contacted surfaces, highlighting realistic domestic transmission pathways. Sensory changes such as odor or discoloration were not correlated with microbial load, indicating that visual assessment is unreliable for sponge replacement decisions. These results underscore the role of kitchen sponges as critical microbial reservoirs in households and emphasize the need for regular sponge replacement or the use of alternative cleaning utensils. The standardized sponge model developed in this study provides a valuable platform for evaluating sanitation strategies and for understanding microbial interactions relevant to domestic hygiene and public health.

Biocides are widely used to control unwanted biological growth. Isothiazolinone biocides are one class of biocides used to stabilize a range of products against microbial contamination and degradation. Although the development of resistance to such biocides is thought to be unlikely, we describe a system capable of detoxifying the common isothiazolinone biocide MIT/CMIT (2-methyl-4-isothiazolin-3-one and 5-chloro-2-methyl-4-isothiazolin-3-one). It was found in Klebsiella oxytoca complex bacteria contaminating a commercial liquid soap that contained the isothiazolinone as a preservative. The isolate inactivates MIT/CMIT via a glutathione-dependent reaction catalyzed by a plasmid-encoded glutathione transferase system. This system comprises a glutathione transferase ( gstA ) and a hydroxyacylglutathione hydrolase ( gloB ), both located in one operon on an 80-kbp plasmid and co-localized with an arsenic resistance cassette. The plasmid can be transferred by conjugation to Escherichia coli . The findings highlight the risk that broadening biocide use can increasingly drive the evolution, selection, and spread of different resistance mechanisms. IMPORTANCE: Biocide usage is increasing worldwide to combat organisms causing spoilage or infectious disease. However, the use of biocides is also recognized as a selection factor that shapes microbial communities and drives adaptation. Isothiazolinone biocides are mainly used to stabilize liquid products, personal care products, and fuel against microbial spoilage. The detoxification system described here allowed bacteria to survive in a liquid soap cleaning product containing the widely used biocide preservative MIT/CMIT. Importantly, the biocide detoxification system is mobile between different species and co-localized with a metal resistance operon. The system thereby demonstrates how the spread and co-selection of resistances can occur in bacterial populations exposed to biocides.