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Environmental stresses increase Listeria resistance

Use of sub-lethal stresses may stimulate antibiotic resistance in L. monocytogenes strains. Photo: CDC
Use of sub-lethal stresses may stimulate antibiotic resistance in L. monocytogenes strains. Photo: CDC

Sub-lethal environmental stresses could stimulate antibiotic resistance in Listeria monocytogenes strains, according to research.

It was found that cold, acid and osmotic stresses increased the resistance of this pathogen to nine currently used antibiotics.

The study evaluated effects of osmotic (2, 4, 6, 12% NaC), pH (6, 5.5, 5.0) and cold (4 °C) stresses on susceptibility of three isolates of L. monocytogenes towards different antibiotics.

L. monocytogenes is exposed to a variety of environmental stresses during food processing and appears able to adapt.

Understanding the effects of stress during food processing on antibiotic susceptibility is important in developing clinical control over pathogens that originate from food, said the researchers.

Effect of salt, cold and pH stresses

When L. monocytogenes cells were exposed to salt, cold and pH stresses, their antibiotic resistance increased as salt concentration increased to 6 or 12%, as pH was reduced to pH 5 or as temperature was decreased to 10 °C.

“The three L. monocytogenes isolates examined were initially sensitive toward the antibiotics tested, but when they were exposed to osmotic, acid or cold stresses, their antibiotic resistance increased,” said the researchers.

“If this represents a broader phenomenon, it may in part explain the emergence of antibiotic resistance among L. monocytogenes isolates from food products.”

Antibiotic treatment of infectious listeriosis usually rely upon the use of β-lactam antibiotics such ampicillin or penicillin, alone or in combination with an aminoglycoside (gentamicin).

However, with patients allergic to β-lactams, trimethoprim and a sulfonamide have been used as an alternative.

During food processing bacteria may encounter conditions that cause chemical (acids, ethanol, alkaline, chlorine and salts) or physical (heat, radiation and pressure) stresses.

The minimal inhibitory concentrations (MICs) of tested antibiotics against unstressed (control), stressed or post-stressed L. monocytogenes isolates (an ATCC strain and a meat and dairy isolate) were determined using the broth microdilution method.

Meat and dairy more resistant

Results showed the meat and dairy isolates were more resistant than the ATCC reference strain.

Three L moncytogenes isolates were challenged with nine antibiotics chosen according to their mode of action and use in clinical therapy.

To examine post-stress conditions on the susceptibility of L. monocytogenes toward tested antibiotics, cells stressed by osmotic, acid, and cold exposure were inoculated into Tryptic Soy Broth (TSB) and incubated at 37 °C for 24 hours.

Then the cells were harvested by centrifugation at 4000g for 20 minutes, the pellets were resuspended and diluted to give a final concentration of about 6 log10 CFU/ml in the reaction mixtures.

Osmotic stress at 2, 4, 6, and 12% NaCl for 24 hours reduced the susceptibility of L. monocytogenes isolates when challenged with ampicillin, tetracycline, doxycycline and vancomycin.

Selected results

With increases in NaCl concentrations, bacterial antibiotic resistance increased to the extent that susceptible isolates became moderately resistant or resistant.

When L. monocytogenes cells were exposed to pH 5.0, all isolates were moderately resistant to penicillin, however, the ATCC strain and the dairy isolate became resistant to streptomycin while the meat isolate became only moderately resistant.

Cold stress (10 °C for 24 hours) increased the resistance of L. monocytogenes toward the tested antibiotics, although there were differences in the extent of the increases which were both strain and antibiotic dependent.

Changes did not appear more extensively with one specific strain or antibiotic; however, the meat and dairy strains tended to show enhanced resistance more often than the ATCC strain.

The increase in antibiotic resistance observed may be related to reduction of cell wall antibiotic binding sites, amplification of genes responsible for efflux pump synthesis and operation, and induction of stress shock proteins.

It was also observed that L. monocytogenes continued to show higher levels of antibiotic resistance after removal of each of the three types of stress.

Source: Food Microbiology Volume 46, April 2015, Pages 154–160

Online ahead of print, DOI: 10.1016/

Effects of osmotic pressure, acid, or cold stresses on antibiotic susceptibility of Listeria monocytogenes”

Authors:  Anas A. Al-Nabulsi, Tareq M. Osaili, Reyad R. Shaker, Amin N. Olaimat, Ziad W. Jaradat, Noor A. Zain Elabedeen, Richard A. Holley

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