ATTACHMENT 4
DAIRY R & D IN SA
by Heinz Meissner
Date: 2015-02-21
CHARACTERIZATION OF ESHERICHIA COLI STRAINS, SOME OF WHICH MAY BE PRESENT IN
BULK MILK.
Esherichia coli is a potential foodborne pathogen which under particular circumstances may
compromise food safety and public health. It is therefore important to identify potential E.
coli pathogens and define the environment in which they operate, including possible
pathogenicity. The objectives in the Aijuka and Buys study were therefore to characterize
environmental E. coli based on virulence and antibiotic resistance which could suggest
pathogenic activity, as well as to determine the genetic relatedness of the isolates.
Esherichia coli isolateswere collected monthly for 10 months from environmental sources
(irrigation water and lettuce). Forty one isolates were exposed to 11 antibiotics at single
concentrations with the Multiple Antibiotic Resistance technique as in humans and animal
husbandry. The same isolates were also tested for presence of virulence genes which may
cause shigatoxins, using real time PCR. Additionally, the isolates were analysed using PCR
fingerprinting to determine genetic relatedness.
The highest resistance to antibiotics was recorded for cephalothin and ampicillin. Twentyfour percent of isolates were positive for at least one virulence gene while 17% showed
positive responses for shigatoxins. Thirty-four percent of isolates were positive for both
antibiotic resistance and virulence. Genetic fingerprinting showed close relatedness (≥80%
similarity) in more than half (>50%) of isolates. This study shows that environmental
sources are reservoirs of potentially pathogenic E. coli which may contaminate the food
chain compromising food safety and public health. Also, many of these strains may be
related.
This study was followed up in a study by Ntuli and co-workers to characterise E. coli
serotypes and their antimicrobial resistance patterns that may occur in bulk milk. This is
important for risk assessment and it is also a critical step for diagnostic and epidemiological
purposes.
A total of 121 E. coli serotypes were isolated and identified from 258 bulk milk samples from
six different geographical regions in South Africa. Two sets of antibacterial agents were used
to evaluate the antibacterial resistance characteristics of the isolates. The first set was
commonly used antibiotics in human medicine and animal husbandry and the second set
was extended-spectrum beta-lactamases antibiotics (ESBL). Antimicrobial resistant E. coli
isolates were detected in 70% of the milk samples, of which 36.8% of the pathogenic E. coli
showed multi-drug resistance. Presence of ESBL capacity was observed in 20.6% of the
isolates. The serotype E. coli O157:H7, which is a dangerous food poisoning organism, was
detected in 25.6% of the isolates. The information from this research should be useful in
modelling the risk from pathogenic E. coli in bulk milk sold in the informal market in South
Africa.
References:
Aijuka, M. & Buys, E.M., 2014. Characterization of E. coli from South African environmental
sources. Proc. of the SASDT Conference 2014, Cape Town.
Ntuli, V., Njage, P. & Buys, E.M., 2014. Antibiotic resistance patterns of identified E. coli
serotypes in bulk milk. Proc. of the SASDT Conference 2014, Cape Town.