Effect of sub-lethal doses of nisin on Staphylococcus aureus toxin production and biofilm formation.

Staphylococcus aureus is one of the commonest food-borne pathogens that can cause gastroenteritis owing to having several enterotoxins. Also, biofilm formation can complicate infections caused by this microorganism. Nisin is a safe food bio preservative which is usually used as an agent to prevent pathogen growth; however, it is important to identify the exact impact of nisin on the growth of S. aureus and to determine the suitable concentration needed for elimination of this pathogen in food. In this study, after MIC determination of nisin against S. aureus ATCC 29213, this strain was treated with sub-MIC (1/2) of nisin (4 µg/ml) and transcript levels of toxin-encoding (hla, SEA, SEB, and SED) and biofilm-associated (fnb, ebpS, eno, and icaA) genes were determined using Quantitative Real-time PCR at 2, 8, and 24 h post exposure. All toxin genes were down-regulated following exposure to sub-MIC of nisin, whereas biofilm-associated genes were up-regulated. The expression levels of fnb and icaA in S. aureus were highest after 8 h (4.5-fold and 6.8-fold increase, respectively), while the expression levels of eno and ebpS genes were highest after 2 h (3.3 and 4.5-fold increase, respectively). According to these results, although transcriptional levels of toxin genes were reduced, sub-MIC concentrations of nisin could trigger the expression of biofilm-associated genes in S. aureus. This can further lead to bacteriocin tolerance such that even its higher concentrations cannot kill bacterial cells after exposure to sub-lethal doses. Therefore, it is pivotal to add appropriate concentrations of nisin to food products for preservation purposes.

The Challenge of Global Emergence of Novel Colistin-Resistant Escherichia coli ST131.

Escherichia coli ST131 is one of the high-risk multidrug-resistant clones with a global distribution and the ability to persist and colonize in a variety of niches. Carbapenemase-producing E. coli ST131 strains with the ability to resist last-line antibiotics (i.e., colistin) have been recently considered a significant public health. Colistin is widely used in veterinary medicine and therefore, colistin-resistant bacteria can be transmitted from livestock to humans through food. There are several mechanisms of resistance to colistin, which include chromosomal mutations and plasmid-transmitted mcr genes. E. coli ST131 is a great model organism to investigate the emergence of superbugs. This microorganism has the ability to cause intestinal and extraintestinal infections, and its accurate identification as well as its antibiotic resistance patterns are vitally important for a successful treatment strategy. Therefore, further studies are required to understand the evolution of this resistant organism for drug design, controlling the evolution of other nascent emerging pathogens, and developing antibiotic stewardship programs. In this review, we will discuss the importance of E. coli ST131, the mechanisms of resistance to colistin as the last-resort antibiotic against resistant Gram-negative bacteria, reports from different regions regarding E. coli ST131 resistance to colistin, and the most recent therapeutic approaches against colistin-resistance bacteria.