Listeria Spp Resistance Mechanisms Biology Essay


Little is known about the mechanisms of resistant bacterium species and their resistance genes isolated in food processing environments, and different food like in Botswana. Bacterial species and strains resistant to different antimicrobial agents are of major public health concern because they pose health risks attributed to human deaths,( Dorota, 2012). Antimicrobial drug resistance is caused by human and non human usage of antimicrobials including treatment of animal diseases (WHO), over the counter prescription and over prescription of antimicrobial drugs in some countries, shortage of health infrastructure, shortage of and unreliability of drug supplies, low quality of drugs (who), and use of microbials to generate plants, food and feed( Dorota et al, 2012).

Furthermore, the occurrence of non-pathogenic bacteria showing resistance to antimicrobial drugs has attained awareness throughout recent years ( [Kastner et al., 2006] and [Talon and Leroy, 2011]). Even et al. (2010). Information and statistics generated from the examination of antimicrobial resistance and antimicrobial handling should play an input in the improvement of public policies for the control of antimicrobial resistance. These information is essential in the pre- and post-licensing process and in the progress and management guiding principles for veterinary use(who).

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Since bacteria have the capacity to develop resistance to antimicrobial agents over prolonged exposure to antimicrobial drugs, it is therefore believed that a Listeria bacterium which is considered to be susceptible to most antimicrobials has the same capability to develop resistance to antimicrobials. Like other medically important bacterial species, Listeria has the capability to pose serious implications to its patients as it results in serious diseases including meningitis in newborn babies, patients with weak or compromised system, and abortion in pregnant women,(morobe et al, 2009, korsak, 2012) .

The continued unregulated use of antimicrobial agents results in natural screening and selection of resistant bacteria strains and antimicrobial resistance genes that can be transmitted between human through food and between bacterial species through natural transfer of plasmids and transposons by conjugation. Plasmids such as pIP501 have been found to be able to replicate in Listeria and to promote its own transfer between strains of Listeria and from Listeria back to Streptococcus (Pérez-Diaz, Vicente 1988), and plasmid pAMβ1 transferable from Enterococcus faecalis, conferring resistance to erythromycin, has been transferred to Listeria. Transposons such as Tn916, previously found in E. Faecalis, carries tet(M) gene causing resistance to tetracycline-minocycline (Vicente 1988).

Bacteria can show resistance to antimicrobial agents using a range of mechanisms. A number of species of bacteria are naturally resistant to 1 class of antimicrobial agents and as such all strains of that bacterial species are resistant to all the members of those antibacterial classes(tenover, 2006). There are cases of acquired resistance, where originally susceptible populations of bacteria become resistant to an antimicrobial drug and grow and spread under the selective pressure of use of that antimicrobial drugs(tenover, 2006).

The bacterium can gain genes encoding enzymes, including lactamases that destroy the antimicrobial drug before it can produce a desired outcome(tenover, 2006). Furthermore bacteria may acquire efflux pumps that remove the antimicrobial drug from the cell before it can get to its target site and produce its outcome. Also bacteria may obtain few genes for a metabolic path which eventually produces changed bacterial cell walls that no longer have the binding site of the antimicrobial agent, or sometimes bacteria can obtain mutations that reduce right of entry of antimicrobial agents to the intracellular target location through down regulation of porin genes. Several genetic mechanisms, including transformation, conjugation, or transduction also confer antimicrobial resistance to drugs. Through genetic exchange mechanisms, many bacteria have become resistant to different classes of antibacterial agents, and such bacteria with multidrug resistance have become a cause for serious alarm, mainly in hospitals and other healthcare facilities where they occur most commonly.

Previous studies have shown that tetracycline resistance is conferred by ribosome protection due to either the tet(M) or tet(S) gene (Charpentier, E., and P. Courvalin. 1999.). The tet(S) gene was detected in both Multi Drug Resistant strains, whereas the remaining strains had acquired tet(M), (morvan). Studies have also shown that flouroquinolone resistance was due to active efflux associated with overexpression of the lde gene(Godreuil,2003).

Previous research have also shown that chloramphenicol resistance is due to acquisition of a cat gene encoding an acetyltransferase which catalyzes acetyl-S-coenzyme A (CoA)-dependent acetylation of chloramphenicol at the 3-hydroxyl group(Poyart-Salmeron, 1990). Streptomycin resistance was attributed to presence of high level BM4210 plasmid encoded by 6-N-streptomycin adenylyltransferase, encoded by the aad6 gene (Charpentier,1999) and suspectedly ribosomal mutations(morvan). Studies have also reported that trimethoprim resistance was due to acquisition of the dfrD gene, encoding a resistant dihydrofolate reductase(Huovinen, 1995). Resistance to trimethoprim has not yet been said to be linked with any antibiotic resistance (morvan)

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Despite the significance and relevance of information on the mechanisms of drug resistant L. monocytogenes strains, there is no information on the antimicrobial resistance mechanisms of L. monocytogenes isolated from food and food-processing environments in Botswana and no extensive studies of this calibre have been carried out. For this grounds, studying the resistance mechanisms of L. monocytogenes, although relatively uncommon, should however be pursued.

Literature review

Statement of the problem

Observation: The reference management and cure of listeriosis is at present based on the use of high doses of ampicillin or amoxicillin and gentamicin because Listeria monocytogenes is highly susceptible to a wide range of antimicrobial drugs. However, the overuse of the above mentioned drugs and other antimicrobial agents over time in animals, plant production and hospitals creates a reservoir of resistant Listeria monocytogenes and of Listeria monocytogenes -borne resistance genes making it difficult to treat listeriosis.

Question: What mechanisms enable Listeria monocytogenes to be resistant to antimicrobial agents although it is known to be highly susceptible to a wide range of antibacterial drugs?


Null hypothesis

Listeria monocytogenes is susceptible to a wide range of antimicrobial agents and as such it has not yet developed mechanisms of resistance to therapeutically used antibiotics.

Alternative hypothesis

Though previously reported to be widely susceptible to a wide range of antimicrobial agents, Listeria monocytogenes has developed mechanisms of resistance to antimicrobial agents.


Listeria monocytogenes resistance to antimicrobial drugs is due to essential genes responsible for non-susceptibility to therapeutically used antibiotics, conjugative transposons and plasmids carrying antibiotic resistance genes and point mutations.

Experimental work:

The experiment will be carried out using Listeria monocytogenes isolates isolated from different food sources around Gaborone (Botswana) in 2009. Based on antimicrobial susceptibility results, tetracycline-resistant L. monocytogenes isolates will be further characterized by detecting the presence of tet(M), tet(K), tet(L), tet(S), and tet(T) genes.