TelA contributes to the innate resistance of Listeria monocytogenes to nisin and other cell wall-acting antibiotics

Abstract

Nisin is a class I bacteriocin (lantibiotic), which is employed by the food and veterinary industries and exhibits potent activity against numerous pathogens. However, this activity could be further improved through the targeting and inhibition of factors that contribute to innate nisin resistance. Here we describe a novel locus, lmo1967, which is required for optimal nisin resistance in Listeria monocytogenes. The importance of this locus, which is a homologue of the tellurite resistance gene telA, was revealed after the screening of a mariner random mutant bank of L. monocytogenes for nisin-susceptible mutants. The involvement of telA in nisin resistance was confirmed through an analysis of a nonpolar deletion mutant. In addition to being 4-fold-more susceptible to nisin, the ΔtelA strain was also 8-fold-more susceptible to gallidermin and 2-fold-more susceptible to cefuroxime, cefotaxime, bacitracin, and tellurite. This is the first occasion upon which telA has been investigated in a Gram-positive organism and also represents the first example of a link being established between a telA gene and resistance to cell envelope-acting antimicrobials.

FIG. 1.

(A) Kinetic growth assay showing impaired growth of EGDe transposon mutant C5 (open symbols) compared to the wild type (WT) (closed symbols) in the presence of 500 μg/ml Nisaplin (2.5% nisin). (B) Deferred antagonism assay showing increased susceptibility of C5 to Nisaplin.

FIG. 2.

(A) Genetic organization of the point of insertion of the himar1 transposon. The dashed line shows the proportion of the gene deleted. The lollipop symbol indicates a putative terminator. All genes in the schematic are drawn to scale. (B) Disk diffusion assay showing a potassium tellurite-impregnated disk on plates spread with wild-type L. monocytogenes and the L. monocytogenes ΔtelA mutant. An increased zone of inhibition indicates increased susceptibility in the ΔtelA mutant. Lighter shades of the zone of tellurite reduction indicate an impaired ability to reduce tellurite to elemental tellurium.

FIG. 3.

Growth of EGDe and the ΔtelA and ΔtelA::telA mutants in the presence of antimicrobial agents. Shown is the growth of EGDe (closed circles), the EGDe ΔtelA mutant (open circles), and the EGDe ΔtelA::telA mutant (closed triangles) in TSB-YE with 100 to 500 μg/ml of Nisaplin (2.5% nisin). Error bars are standard deviations from the means of data from triplicate experiments.