TOXIN FORMATION BY STAPHYLOCOCCUS AUREUS IN MILK AND MILK PRODUCTS IN ZARIA AREA AND ITS PUBLIC HEALTH SIGNIFICANCE
TOXIN FORMATION BY STAPHYLOCOCCUS AUREUS IN MILK AND MILK PRODUCTS IN ZARIA AREA AND ITS PUBLIC HEALTH SIGNIFICANCE
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Date
1988-03
Authors
VERONICA, JARLATH UMOH (MRS)
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Abstract
Staphylococci were isolated from fermented milk
(nono), fermented milk-cereal mix (fura), local
butter (manshanu), raw milk, whole milk and
commercial yoghurts using Baird-Parker agar as the
selective medium. Of the 197 samples of "nono"
purchased from 4 local markets around Zaria, 346
staphylococcal isolates were obtained and only
19(9.6%) of the samples were free of staphylococci.
Ninety-three samples of "fura" and 79 samples of
"manshanu" yielded 197 and 25 staphylococcal isolates
and 1(1.1%) and 49(62.0%) of the samples were
negative for staphylococci respectively. The mean pH
of "nono" and "fura" ranged from 3.81 to 4.28 and
titratable acidity (TA) ranged from 0.56 to 0.67. The
total aerobic plate count (TAPC) of "nono", "fura"
and "manshanu" ranged from 6.7xl04 to 2.7xl06 cfu/ml
and staphylococcal count (SC) from 3.4x102 to 4.3xl04
cfu/ml. About 95% of the longlife whole milk, milk
drinks and commercial yoghurts were without
detectable staphylococcal growth.
Of all the staphylococcal isolates from the
locally fermented milk products, 37(6.5%) were
enterotoxigenic, made up of 21 from "nono", 16 from
"fura" and none from "manshanu". Staphylococcal
enterotoxin A (SEA) was produced by 73.0% of the
enterotoxigenic strains, 10.8% produced SEB and SEC
each and 2.7%
produced both SEA and SEB and both SEA and SEC each.
The cat bioassay did not reveal any new enterotoxin
production by 35 coagulase and thermonucleasepositive
staphylococci earlier demonstrated to be
negative for enterotoxins A through E.
Forty-two staphylococcal isolates were obtained
from 135 California Mastitis test (CMT)-positive and
CMT-negative raw milk samples from settled Government
cattle herds and 8(19.0%) were enterotoxigenic,
5(11.9%) produced SEA and 3(7.1%) produced SED. None
of the isolates obtained from nomadic herds was
enterotoxigenic.
Four hundred and eighty fermented milk products
collected over a 12-month period from two markets
showed that SC and TAPC were significantly higher in
the wet season and low in the dry harmattan season.
Titratable acidity was significantly lower between
March and May (late dry period). There was no
significant difference in pH of the samples for the
various seasons.
Chemical analysis shows that when the proximate
composition of "nono" was compared to other fermented
milk products it was low as the fat content ranged
from 0.08 to 0.68%, total solids from 3.15 to 6.17%,
total protein from 1.21 to 1.90%, solid-non-fat, 3.07
to 5.42% and ash 0.24 to 0.37%. Commercial yoghurts
had proximate composition comparable to the
standards of other countries.
Bacteriophage typing of 147 S.aureus isolates
using International human phage set, revealed that
from fermented milk and raw milk, 54(66.7%) and
25(37.9%) of the isolates were typable respectively.
Majority of the phages were typed as phage lytic
group III and mixed type with lytic phages 42E, 75
and 85 predominating. Out of 110 isolates typed with
bovine phage set, 29(44.6%) were typable and these
were obtained from fermented milk and 23(51.1%)
obtained from raw milk, most of which were bovine
phage group III, IV and mixed type. The strong lytic
phages were 42E, 75 and 102.
The resistance patterns of staphylococcal
isolates from raw milk and fermented milk products to
8 antimicrobial agents were studied. The minimum
inhibitory concentration using the tube assay of the
isolates resistant by the disc method were > 16ug for
ampicillin in 5(11.9%) isolates, > 12 units for
penicillin in 12(22.2%) isolates, > 40ug in 9(36.0%);
9(90.0%); 9(69.2%); 8(72.7%) and 7(70.0%) of isolates
for cloxacillin, chloramphenicol, streptomycin,
erythromycin and tetracycline respectively.
The effect of storage temperatures of 4°C,
12°C, 30°C and 37°C and pH values of 3.0, 3.5, 4.0,
4.5 and 5.0 on two strains of S.aureus (F265 and B46)
showed that the two strains were equally sensitive to
low pH of 3.0 to 4.0. At pH higher than 4.0 and a
temperature of between 12°C to 37°C, there was a
slight growth in the first 8 hours followed by a
decline probably due to combined effect of pH,
limited nutrients and competition with lactic acid
bacteria.
Sensory analysis showed that the quality and
flavour of "nono" declined after 3 days of storage at
4°C and at ambient temperature. Enterotoxin assay of
the product inoculated with enterotoxin A and D
producing strain of S.aureus (Z88) and stored for 3
days at ambient temperature showed that the hazardous
level of staphylococci (106 cells/ml) was maintained
for the first 4 hours followed by a decrease to 102
cells/ml within 72 hours. However, assay for
enterotoxin A in these products were negative by the
microslide technique.
The high degree of contamination of the local
products examined calls for hygienic methods of
handling these dairy products. The use of prepared
starter could help prevent starter failure, hasten pH
decrease and reduce the risk of growth and
enterotoxin production by contaminating strains.
Phage typing result indicates a post-process
contamination of the fermented milk by human strains
of S.aureus. The raw milk were contaminated with
bovine strains. Therefore simple packaging of the
fermented products might help solve this problem
since the bovine strains were probably killed during
processing.
Description
A Dissertation Submitted to the Postgraduate
School Ahmadu Bello
University, Zaria, in partial fulfillment of the
requirements for the degree of Doctor of Philosophy
in
Microbiology
Department of Microbiology, Faculty of Sciences,
Ahmadu Bello University, Zaria.
Keywords
TOXIN,, FORMATION,, STAPHYLOCOCCUS,, AUREUS,, MILK,, PRODUCTS,, ZARIA,, PUBLIC,, HEALTH SIGNIFICANCE.