The genome of Salmonella enterica possesses multiple pathogenicity islands (PIs), which are genetic elements within the bacterial genome that harbor genes associated with virulence [9]. For our experiments, five virulence genes for PCR amplification from the Salmonella serovar were selected.
The invA genes contain sequences that are unique to Salmonella spp. and have been shown to be suitable for specific targets in different diagnostic and research laboratories [10, 11]. Several studies in Africa have shown similar results [12]. In South Africa, Afema et al. (2016) found similar results from treated wastewater used for the irrigation of vegetables.
The misL gene encodes an auto-transporter protein involved in intestinal colonization and essential for survival in macrophages [13]. The majority of Salmonella isolates in this study carried this gene. Similar results were found in most studies in South Africa and Colombia [14, 15].
Recent studies have shown that caspase-1 apparently has a protective role for the host during systemic Salmonella infection, suggesting that caspase-1 activation by Salmonella would be detrimental to the organism in disseminated disease [16, 17].
The spvR locus is strongly associated with strains that cause non-typhoid bacteremia, but are not present in typhoid strains [13]. SpvR is a positive regulator gene of four effector genes, spvA, spvB, spvC and spvD. These virulence genes are found in the most frequently isolated non-typhoid serotypes of S. enterica. Indeed only Salmonella Typhimirium and Enteritidis could contain the spv plasmids of virulence, which explains why bacteria carrying this plasmids can’t cause gastroenteritis in people [13]. However, this study showed spvR in 61.9% of Salmonella Typhi and Paratyphi isolates from human diarrheal samples. The presence of these virulence genes in Salmonella Typhi and Paratyphi isolated from lettuce and clinical samples indicate the capabilities of these isolates in causing infections in susceptible hosts.
Resistances related to class 1 integrons were found in 40%. Class 2 and 3 integrons were absents. This predominance of class 1 integrons in Salmonella was previously described by others authors [18–21]. According to [22] the prevalence of integrons found in Salmonella varies from country to another country and depends on the origin of the isolates. Class 1 integrons were found to 35%; 28%; 46%; 56.72% in Vietnam, England, Kenya and Brazil respectively [22]. Sequencing of the gene cassette showed that most of our strain carried aadA1 and dfrAI. Others studies realized in Africa (Senegal, Kenya, Egypt) were showed similar results [21, 23–25]. Resistance to trimethoprim/sulfamexazole is strongly associated with the presence of class 1 integrons, due to the frequent presence of a dfrA1 cassette and sul1 gene in the 3’ region [26].
Integrons are potentially capable to transmit drug resistance to other S. enterica isolates or to other bacteria. Since integron represents the main vehicle of antibiotic resistance, their presence in S. Typhi indicates uninterrupted transfer of drug resistance genes from one organism to another irrespective of their species which are worrisome with respect to the spread of AMR [27].
Class 1 integrons are increasingly described in environmental and animal bacteria. The release of hospital and municipal effluents is the main way to integrate integrons into the environment [22]. In this study according to our methodology, transfer experiments were not successful. It is possible that these class 1 integron which carried resistance genes contained in Salmonella isolated were not a transferable elements.
The main mechanism of resistance to quinolones is linked to chromosomal mutations especially in the gyrA or parC genes and more rarely at the gyrB and parE genes [28, 29]. This study showed only 8 isolates could be amplified both the gyrB and parE genes. Previous studies from Brazil and Senegal were showed similar results with our results [28–31].
Extrachromosomal genes qnr, aac(6')-Ib-cr and qepA were discovered since 2002 and are carried by conjugative plasmids [32]. Since then, several types of qnr (qnrA, qnrB, qnrS) have been described [32]. These genes were not detected in this study. Several studies shown that these plasmids were most found in non-typhoidal Salmonella and absent in Salmonella Typhi. This was similar with ours results. Others studies shown that plasmids carrying genes encoding ESBLs generally carry genes encoding quinolone resistance [33, 34]. This study showed only one ESBL-producing strain (blaCTX-M-14-like). This results were similar to [35] in Sub-Saharan African from blood samples to febrile patients.
Pulsed-field gel electrophoresis has been widely used to determine strain relatedness, to confirm bacterial disease outbreaks, and to identify the sources of strains [36, 37]. In the present study, three distinct PFGE types (pulsotypes) were observed from clinical samples with 90–95% similarity in each case. The diversity of pulsotypes could explained by the fact that the samples come from different zones and the fact that the clinical isolates were collected from much longer time span, so the PFGE patterns would change more. These zones differ from their climatic, socio-cultural and even demographic conditions. However all Salmonella Paratyphi B from lettuce samples indicated indistinguishable pulsotypes. This is not surprising since these isolates originate from the same sampling site. In addition, 80–90% similarity was observed between these pulsotypes and clinical ones from Ouagadougou. This could be due to the fact that our strains from lettuce were collected in gardens close to a health center.