Food Protection Trends - January/February 2020 - 11

to calculate the CFU per cm2. Estimated surface areas
were: trolley handles, 132 cm2; trolley grill, 2903 cm2; and
bottom of handheld baskets, 892 cm2. Means, standard
deviations, and medians were calculated using Microsoft
Excel version 16.25. Mean values were used to analyze
findings for each microorganism and surface. With regard
to pathogen detection, proportions of each microorganism
were calculated by considering the number of positive results
relative to the total number of samples analyzed per surface.

and the lowest on trolley grills (2.40 × 102, 0.27, and 0.03
CFU/cm2, respectively).
With regard to environmental Listeria, the results are
presented as the proportion of samples positive for the
genus. A total of 76% (74 out of 96, which comprise the
three surfaces) of the tested samples were positive for Listeria
spp. The highest incidence was observed on the bottom
of handheld baskets, with 97% of the samples positive for
Listeria spp. (24 out of 25), while a total of 69% of trolley
grills (25 out of 36 samples) and 69% of trolley handles (25
out of 36 samples) were found to carry Listeria spp. Further
investigation of the non-pathogenic species revealed that L.
innocua, L. grayi, L. welshimeri, L. seeligeri, and L. ivanovii were
present on the surfaces, with a distribution of 53, 4, 26, 7, and
2%, respectively.
With regard to the bacterial load of the studied indicator
organisms present on each surface, trolley grills consistently
had the lowest concentration of all microbial indicators
(APC, YM, EB, CF, and EC), and trolley handles carried the
highest microbial concentration of APC, EB, CF, and EC. On
the other hand, YM was highest on the bottom of handheld
baskets, the surface that also had the highest proportion of
samples positive for Listeria spp.

RESULTS
Indicator organisms
Microbial indictors provide a general idea of the current
hygienic conditions and an indirect measure of potential
pathogens present. APC and YM give a measure of the
cleanliness of the surfaces, while CB, EB, and CF suggest
the possible presence of pathogenic members of E. coli,
Salmonella, Klebsiella, and Cronobacter, among others.
Listeria spp. is typically used as an indicator of the potential
presence of L. monocytogenes. Tables 1, 2, and 3 show the
level at which each indicator microorganism was present
on the three surfaces tested. With respect to APC and YM,
findings indicate their presence on every sample tested. The
greatest APC load was found on trolley handles (1.25 × 102
CFU/cm2), the second greatest on the bottom of handheld
baskets (1.0 × 102 CFU/cm2), and the lowest on trolley
grills (4.94 CFU/cm2), while YM was at the highest level
on the bottom of handheld baskets (1.0 × 103 CFU/cm2)
and at much lower levels on trolley handles and grills (5.5
and 1.28 CFU/cm2, respectively).
With regard to the group of EB, CF, and EC, the greatest
level of contamination was found on the trolley handles
(2.72 × 104, 2.71 × 106, and 27.73 CFU/cm2, respectively),
the second greatest on the bottom of handheld baskets
(4.67 × 103, 3.52 × 105, and 5.07 CFU/cm2, respectively),

Bacterial pathogens
E. coli O157, Salmonella sp., L. monocytogenes, and S. aureus
were found on trolleys and grocery baskets (Fig. 2). Trolley
handles tested positive only for E. coli O157 and S. aureus,
while trolley grills tested positive for both of these pathogens
as well as Salmonella. In contrast, the bottom of handheld
baskets tested positive for all four pathogens analyzed (E.
coli O157, S. aureus, Salmonella, and L. monocytogenes). S.
aureus, the most prevalent pathogen, was found at different
proportions on all of the surfaces tested. Staphylococcus spp.
was found on 81% of the samples (78 out of 96 samples

TABLE 1. Indicator microorganisms on trolley grills*
Average** CFU/cm2

Standard deviation

Median CFU/cm2

Range CFU/cm2

Aerobic Plate Count

4.94

6.6

3.5

0.29 - 36.3

Yeast and Molds

1.28

1.4

0.8

0.1 - 5.61

Enterobacteriaceae

2.40 × 102

1.4 × 103

0.03

0.01 - 8.6 × 103

Coliforms

0.27

1.2

0.01

0.01 - 7.28

Escherichia coli

0.03

0.1

0.00

0.00 - 0.86

*Sample size n = 36.
**Value corresponds to the average of microbial concentration of each indicator microorganism, and the CFU/cm2 was estimated
based on the surface area of the trolley grills.
Aerobic Plate Count = APC; Yeast and Molds = YM; Enterobacteriaceae = EB; Coliforms = CF; Escherichia coli = EC.

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Food Protection Trends - January/February 2020

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