Bovine NK-lysin peptides exert potent antimicrobial activity against multidrug-resistant Salmonella outbreak isolates

Comparative evaluation of antimicrobial activity of human granulysin, bovine and porcine NK-lysins against Shiga toxin-producing Escherichia coli O157:H7

Shiga toxin-producing Escherichia coli (STEC) O157:H7 (O157) is a foodborne pathogen causing human disease ranging from hemorrhagic colitis and hemolytic uremic syndrome to kidney failure, while remaining harmless to cattle, its primary reservoir. The severity of the human disease associated mainly with Shiga toxin production and a global emergence of antibiotic resistant STEC highlights the need for effective non-antibiotic, pre-harvest strategies to reduce O157 in cattle, the principal source of human infection. Towards this goal three synthetic antimicrobial peptides (AMPs): human granulysin (hGRNL), bovine NK-lysin (bNK2A), and porcine NK-lysin (pNKL), were tested in vitro against O157 isolates. As expected, circular dichroism spectroscopy findings were consistent with a predominantly α-helical conformation for all three AMPs in an environment mimicking bacterial outer surface or liposaccharides. The minimum inhibitory concentrations (MIC) and minimum bactericidal concentrations of hGRNL (200 μM), bNK2A (12.5 μM against strain 86-24 and 25 μM against EDL933), and pNKL (6.25 μM) were determined using the Clinical and Laboratory Standards Institute broth microdilution method in Müeller-Hinton broth (cation-adjusted). The bNK2A and pNKL AMPs did not induce Shiga toxin expression in O157 at MIC, as there was a significant decrease or no change in toxin expression following 4- or 20 h incubation with the AMPs; bNK2A p <0.0001 (4 h) and p = 0.4831 (20 h); pNKL p <0.0001 (4 h) and p = 0.0001 (20 h). Propidium iodide uptake assay revealed faster O157 membrane damage or killing kinetics with bNK2A and pNKL compared to hGRNL. Nonetheless, transmission electron microscopy demonstrated that all three AMPs mediated damage to O157 membranes. In contrast, the three AMPs showed minimal cytotoxicity (<2%) against cattle red blood cells at tested concentrations (0.39-50 μM). Overall, our results demonstrate the potential for bNK2A and pNKL to be further developed into novel non-antibiotic agents to reduce O157 shedding in cattle.

Increasing antimicrobial susceptibility of MDR Salmonella with the efflux pump inhibitor 1-(1-Naphthylmethyl)-piperazine

Salmonella is a widespread foodborne pathogen that can exhibit multidrug resistance (MDR; resistance to ≥3 antimicrobial classes). Therefore, the development of new preventative measures against MDR Salmonella is highly important. Bacterial antibiotic resistance is commonly mediated by efflux pumps. In this study, two compounds that block efflux pump activity, 1-(1-Naphthylmethyl)-Piperazine (NMP) and Phenylalanine-arginine β-naphthylamide (PaβN), were tested with the antibiotic tetracycline to determine if a synergistic reduction in resistance could be achieved in tetracycline-resistant Salmonella. The efflux pump inhibitors (EPIs) reduced Salmonella resistance to tetracycline by 16 to 32-fold in several tetracycline resistant isolates. For example, the tetracycline minimum inhibitory concentration (MIC) for MDR Salmonella enterica serovar I 4,[5],12:i:- USDA15WA-1 (SX 238) was 256 μg/mL. However, in the presence of NMP (250 μg/mL), the MIC dropped to 8 μg/mL which is below the Clinical Laboratory Standards Institute (CLSI) breakpoint for tetracycline resistance in Salmonella (≥16 μg/mL). Confocal and transmission electron microscopy revealed NMP-mediated damage to Salmonella membranes at a higher concentration (1000 μg/mL), implying that the EPI disrupts membrane morphology which can lead to cell death; however, this effect was dependent on NMP concentration, as NMP blocked efflux activity with less of a membrane-disrupting effect at a lower concentration (250 μg/mL). These findings suggest that the use of EPIs can reduce the MIC of tetracycline and restore the effectiveness of the antibiotic against tetracycline-resistant Salmonella.

Exposure of Salmonella enterica serovar 1,4,[5],12:i:- to benzalkonium chloride leads to acquired resistance to this disinfectant and antibiotics

AIMS

Disinfectants such as benzalkonium chloride (BC), extensively used in animal farms and food-processing industries, contribute to the development of adaptive and cross-resistance in foodborne pathogens, posing a serious threat to food safety and human health. The purpose of this study is to explore whether continuous exposure of Salmonella enterica serovar 1,4,[5],12:i:- (S. 1,4,[5],12:i:-) to sublethal concentrations of BC could result in acquired resistance to this agent and other environmental stresses (e.g. antibiotics, heat, and acid).

METHODS AND RESULTS

BC tolerance increased in all tested strains after exposure to gradually increasing concentrations of BC, with increases in minimum inhibitory concentrations between two and sixfold. The survival rate of BC-adapted strains was significantly (P < 0.05) higher than that of their wild-type (non-adapted) counterparts in lethal concentrations of BC. In addition, significant reductions (P < 0.05) in zeta potential were observed in BC-adapted strains compared to wild-type ones, indicating that a reduction in cell surface charge was a cause of adaptative resistance. More importantly, two BC-adapted strains exhibited increased antibiotic resistance to levofloxacin, ceftazidime, and tigecycline, while gene mutations (gyrA, parC) and antibiotic efflux-related genes (acrB, mdsA, mdsB) were detected by genomic sequencing analysis. Moreover, the tolerance of BC-adapted strains to heat (50, 55, and 60°C) and acid (pH 2.0, 2.5) was strain-dependent and condition-dependent.

CONCLUSIONS

Repeated exposure to sublethal concentrations of BC could result in the emergence of BC- and antibiotic-resistant S. 1,4,[5],12:i:- strains.

A saposin domain-containing protein of tongue sole Cynoglossus semilaevis: Antimicrobial activity and mechanism

Prosaposin is a precursor that can be processed into four different saposins, designated as A, B, C, and D, which have multiple functions in mammals, including neuroprotection and immune modulation. The immune function of saposin in teleost remains largely unknown. In the present study, a saposin (SAP) domain-containing protein was identified in half-smooth tongue sole Cynoglossus semilaevis and named CsSDP. CsSDP harbors one SAP A domain and two SAP B domains. When expressed in HEK293T cells, CsSDP was specifically localized in the lysosome. When overexpressed in Escherichia coli, CsSDP markedly inhibited bacterial growth, and the inhibitory effect depended on two specific regions in the SAP A and SAP B domains. Two polypeptides (P32 and P30) derived from the above SAP A and B domains could bind to and inhibit the growth of both Gram-negative and Gram-positive bacteria. The ultrastructural analysis revealed that P32 and P30 killed target bacteria by disrupting the bacterial cell wall and inducing substantial release of cytoplasmic contents. These results shed new lights on the immune function of saposin domain-containing protein in teleost.