Salmonella enterica subsp. enterica formed the major group that represents nearly
60% of the salmonellae. Salmonella organisms emerged as a public health problem
in many countries as salmonellosis has become the most prevalent foodborne disease
worldwide. It has been estimated that approximately 1.4 million cases were reported
annually in the developed nations such as USA. In Malaysia, of 8,640 cases of food
poisoning reported by the Ministry of Health for the year 1999, 811 (9.4%) were due
to Salmonella. The purpose of this study was to characterize and study Salmonella
enterica subsp. enterica (S. enterica) using multiple antimicrobial resistance and
several molecular typing methods including plasmid profiling, PCR-RFLP, RAPD,
ERIC-PCR and Multiplex PCR on antibiotic resistant gene. The isolate were
recovered from poultry meat (55), four types of indigenous vegetables namely
‘selom’ (Oenanthe stolonifera) (59), ‘pegaga’ (Centella asiatica) (20), ‘kesum’
(Polygonum minus) (41), ‘kangkong’ (Ipomoea aquatica) (14) and processed food
(11).Genomic DNA of the 200 S. enterica isolates belonging to 43 different serovars were
recovered from poultry meat, various indigenous vegetables and processed food was
confirmed by specific and duplex PCR targeting the iroB gene that yielded 443 bp
and 606 bp amplicons. The PCR amplification of iroB gene is a rapid and reliable
method for distinguishing between S. enterica and other bacterial species.
Plasmids of S. enterica varied in sizes from 2 to more than 200 kb. Despite limited
knowledge on their function, their presence is frequently used for strain
differentiation in epidemiological studies. Plasmid profiling on the 200 S. enterica
isolates demonstrated high discriminatory capability for serovars differentiation in
this study that was clustered into 70 groups based on the number and pattern of the
bands.
One of the amplification based techniques used in this study for molecular
characterization was PCR-RFLP that incorporated PCR of iroB1, iroB2 and
restriction digest with BglII and AluI to determine the relatedness of bacterial strains.
Results obtained showed that PCR-RFLP has excellent typeablity but low
discriminatory power due to its inability to produce different banding patterns.
ERIC sequences are short, highly conserved 126 bp non-coding regions found in the
Enterobacteriaceae. Its location in bacterial genomes allows discrimination at the
genus, species and serovars levels. RAPD is an amplification-based technique using
arbitrary primers to detect changes in the DNA sequence at the sites in the genome
and enable the discrimination of samples according to sources and serovars.
Dendrogram of RAPD and ERIC-PCR were analyzed and comparisons made using BioNumerics gel analysis software (Applied Maths, Kortrijk, Belgium). Among the
200 isolates of S. enterica, RAPD with arbitrary primers OPAR02, OPAR17 and
OPAR19 generated 47 clusters and 13 single isolates whereas ERIC-PCR with
primers ERIC-1 and ERIC-2 produced 46 clusters and 12 single isolates at 60%
similarity level with discriminatory index (D) of 0.9726 and 0.9606 respectively.
Composite analysis of RAPD and ERIC-PCR profiling simultaneously produced 50
clusters and 18 single isolates at 60% similarity level with highest discriminatory
index of 0.9824. These results demonstrated that composite analysis of RAPD
(OPAR02, OPAR17 and OPAR19) together with ERIC-PCR are a better tool for
differentiation and characterization of S. enterica as compared to a single method
approach.
The multiplex PCR targeted three different antibiotic resistance genes that was used
to detect TEM, PSE-1 and cmlA/tetR genes segment encoding resistance towards
ampicillin, chloramphenicol and tetracycline, respectively which could reduce labour
and cost in analysis of a large number of isolates.
Subsequently antimicrobial resistance was performed using disc diffusion method
with a selection of 13 different antimicrobial agents. Total of 66 profiles were
generated and multiple antimicrobial resistance (MAR) analysis indicated poultry
meat still remains as the main reservoir for multi drug resistant Salmonella. In
contrast, six isolates from the indigenous vegetables showed the highest MAR index
(0.69). This might be due to animal waste fertilizer, irrigation water, contaminated
container and improper handling of food by human that contributed to be the sources
of Salmonella contamination of vegetables. Further investigations need to be conducted to determine if Salmonella isolates in recovered from indigenous
vegetables were gaining more antimicrobial resistance. The characterization of MAR
enabled the determination of antimicrobial patterns and trends in Salmonella from
poultry meat and indigenous vegetables in Malaysia.
As a conclusion, the results from this study could provide valuable information on
the epidemiology and drug resistance trends of S. enterica, and hence contribute
towards better surveillance and infection control measures as well as improved
public health policy