Bile-Mediated Aminoglycoside Sensitivity in Lactobacillus Species Likely Results from Increased Membrane Permeability Attributable to Cholic Acid

Abstract

Few studies have been conducted on antimicrobial resistance in lactobacilli, presumably because of their nonpathogenic nature as anaerobic commensals. We assessed resistance in 43 type strains and isolates representing 14 species by using agar disk diffusion and MIC analysis in MRS medium. Most noteworthy were two general phenotypes displayed by nearly every strain tested: (i) they were more susceptible (up to 256-fold in some cases) to the deconjugated bile acid cholic acid than to the conjugate taurocholic or taurodeoxycholic acid, and (ii) they became susceptible to aminoglycosides when assayed on agar medium containing 0.5% fractionated bovine bile (ox gall). Two-dimensional MIC analyses of one representative strain, Lactobacillus plantarum WCFS1, at increasing concentrations of ox gall (0 to 30.3 mg/ml) displayed corresponding decreases in resistance to all of the aminoglycosides tested and ethidium bromide. This effect was clinically relevant, with the gentamicin MIC decreasing from >1,000 to 4 g/ml in just 3.8 mg of ox gall per ml. In uptake studies at pH 6.5, [G-3 H]gentamicin accumulation increased over control levels when cells of this strain were exposed to bile acids or reserpine but not when they were exposed to carbonyl cyanide m-chlorophenylhydrazone. The effect was dramatic, particularly with cholic acid, increasing up to 18-fold, whereas only modest increases, 3-and 5-fold, could be achieved with taurocholic acid and ox gall, respectively. Since L. plantarum, particularly strain WCFS1, is known to encode bile salt hydrolase (deconjugation) activity, our data indicate that mainly cholic acid, but not taurocholic acid, effectively permeabilizes the membrane to aminoglycosides. However, at pHs approaching neutral conditions in the intestinal lumen, aminoglycoside resistance due to membrane impermeability may be complemented by a potential efflux mechanism.

Antibiotic Resistances of Starter and Probiotic Strains of Lactic Acid Bacteria

“…In addition, it was shown that when lactobacilli were grown in medium containing bile, they became more sensitive to aminoglycosides, suggesting that membrane impermeability plays an important role in this intrinsic aminoglycoside resistance (9). Our results also showed that the LAB are intrinsically resistant when the SCAN or FEEDAP MIC breakpoints are used (Table 4).…”

Dissecting the regulation of bile-induced biofilm formation in Staphylococcus aureus

“…S2), we proceeded to test the MIC of gentamicin, streptomycin, neomycin and kanamycin in the presence of 1 mM SC. The MIC of all aminoglycosides was reduced in the presence of 1 mM SC (Table 3), consistent with a previous study in Lactobacillus species (Elkins & Mullis, 2004), where the authors implicated increased antibiotic uptake following bile exposure.…”

Mammalian Steroid Hormones Are Substrates for the Major RND- and MFS-Type Tripartite Multidrug Efflux Pumps of Escherichia coli

“…As with bile acids, reports of bacterial interactions with steroid hormones are limited to the hydrolysis of such conjugates (deconjugation) and transformations of the steroid ring structure by several genera of gram-positive commensal bacteria (2). The significance of these activities to the bacterium remains unclear but may involve sequestering substrates for assimilatory or metabolic pathways (22) or providing a mechanism to modulate membrane permeability (4). Thus far, these activities have been attributed to a gram-negative organism in only a few cases, which, interestingly, implicated E. coli in the hydrolysis of steroid glucuronides (8) and the 7␣-dehydroxylation of bile acids (23).…”

“…3). Thus, estradiol uptake was monitored at increasing concentrations of either conjugated (taurocholic) or deconjugated (cholic) bile acid within GI tract physiological ranges of Ͻ10 mg/ml (approximately 18 to 20 mM) supplied exogenously in the assay medium during the preincubation period of the (4). Exogenous hydrocortisone, like exogenous taurocholic and cholic acids, failed to significantly modulate estradiol uptake, unlike exogenous progesterone, which provided the largest modulation and produced a biphasic profile.…”

“…An uptake assay was performed essentially as described previously (4). Briefly, cultures were grown to mid-log phase (approximately 4 to 5 h), harvested by centrifugation, resuspended in 0.5 volume of LB broth, and normalized by turbidity analysis.…”