A deteriorating trend has been noted in the bacteriological quality of surface irrigation water
sources in South Africa. In a bid to compare the bacteriological quality of two irrigation
water sources as well as whether irrigation water was a source of bacterial pathogens on
irrigated lettuce, this study was designed and divided into two phases. Phase one involved
determination of physico-chemical parameters and bacterial indicators in the Loskop canal,
the Skeerpoort river and lettuce irrigated with water from the Skeerpoort river over 10
months. Co-currently the study further determined the diversity of the most prevalent
bacterial microflora in the 3 sample sources over the same time period.
Aerobic colony counts (ACC), Aerobic spore formers (ASF), Anaerobic spore formers
(AnSF), Faecal coliforms (FC), Intestinal enterococci (IE) and Staphylococcus aureus (S.
aureus) as well as prevalence of Escherichia coli (E. coli), Salmonella spp and Listeria
monocytogenes (L. monocytogenes) were determined. Additionally the most prevalent
aerobic bacterial species isolated from the three sources were determined. Higher mean
rainfall was noted in areas surrounding the Skeerpoort river (74.7mm) than the Loskop canal
(0.1mm). Mean temperature was 15.4˚C and 18.2˚C while mean pH was 7.4 and 8.4 in the
Loskop canal and the Skeerpoort river respectively. Low mean bacterial counts of less than
3.4 log10cfu/ml, were noted for ACC, ASF, AnSF, S. aureus and IE at both irrigation sites.
Higher mean ACC of 5.9 log10cfu/g and S. aureus counts of 3.0 log10cfu/g were noted on lettuce. Although low mean counts of FC (1.3 log10cfu/100ml) were noted for all three
sources, high incidence of E. coli was observed during bacterial composition studies on nonselective
media. This suggested underestimation of faecal contamination possibly indicating
that identification of specific pathogens provided a better measure of assessing bacterial
contamination than bacterial indicators. E. coli, Bacillus spp and Enterobacter spp were the
most prevalent bacteria in the Loskop canal, the Skeerpoort river and on lettuce. Prevalence
of E. coli, Bacillus spp and Enterobacter spp in the Loskop canal was 23%, 33% and 26%
respectively. Similarly prevalence in the Skeerpoort river was 36%, 26%, 16% respectively.
On lettuce prevalence of the same bacteria was 36%, 30% and 6% respectively. E. coli
O157:H7 was isolated at both irrigation sites while Salmonella enterica (gp 1) ST paratyphi
A was isolated from the Skeerpoort river. High prevalence of similar bacterial species within
the Loskop canal and the Skeerpoort river suggested similar sources of contamination in the
two water sources inspite of different geographical location and surrounding land use
practices. Additionally, similar bacterial species in irrigation water from the Skeerpoort river
and on irrigated lettuce suggested water as a source of contamination on produce.
Additionally it suggests ability of bacterial pathogens to withstand environmental conditions
under field conditions which may pose a risk to food safety and public health among
individuals consuming irrigated fresh produce.
Phase 2 aimed at determining the prevalence of antibiotic resistant and virulent E. coli
collected from the Loskop canal, the Skeerpoort river and lettuce irrigated with water from
the Skeerpoort river. Forty one (41) E. coli isolates: (19) Loskop canal; (12) the Skeerpoort
river; (10) lettuce were tested with 11 antibiotics at single concentrations and screened for
Shigatoxin 1 (stx 1), Shigatoxin 2 (stx 2) and intimin (eae) genes. Antibiotic resistance was
also used as a means of clustering E. coli isolated from the 3 sources. In the Loskop canal
84% and 83% of strains in the Skeerpoort river were resistant to at least one antibiotic. There
was a significant difference (p≤0.05) in resistance to antibiotics between isolates from the
Loskop canal and the Skeerpoort river. Additionally the combined effect of isolate source
(irrigation water site) and antibiotics for isolates from the Skeerpoort river was significant
(p≤0.05). From lettuce, 90% of isolates were resistant to at least one antibiotic and resistance
significantly differed (p≤0.05) from isolates in the Skeerpoort river. The highest resistance to
single antibiotics in all three samples was to cephalothin and ampicillin. Higher resistance
was noted to multiple (more than 2) antibiotics in the Skeerpoort river (33%) than Loskop
canal (5%). Most isolates from the same source showed close relatedness.
Close relatedness was noted between isolates from the Loksop canal (10.5%) and the
Skeerpoort river (16%). From irrigated lettuce 40% of isolates showed close relatedness to
isolates in irrigation water from the Skeerpoort river. In the Loskop canal 15% and 41% of
isolates in the Skeerpoort river possessed virulence genes. From lettuce, 20% of isolates
possessed virulence genes. In the Loskop canal as well as from lettuce all isolates with
virulence genes were antibiotic resistant while 80% of isolates with virulence genes in the Skeerpoort river were antibiotic resistant. In the Loskop canal 10% and 25% of isolates in
the Skeerpoort river were positive for stx1/stx2 and eae, genes synonymous with
Enterohaemorrhagic E. coli (EHEC). Results from this study show that E. coli from the two
irrigation water sources as well as on irrigated lettuce were resistant to antibiotics and
potentially pathogenic. This may increase risk of contaminating irrigated fresh produce which
may compromise food safety and public health of consumers.