Analysis of Microbial Load And Diversity in Crickets (Gryllus Bimaculatus and Scapsipedus Marginatus) Used as A Source of Protein for Food
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Date
2019-04
Authors
Jedida Wanjiru, Gatheru
Journal Title
Journal ISSN
Volume Title
Publisher
Kenyatta University
Abstract
Crickets are drawing interest as sustainable source of protein for food and feed
worldwide. However, there is little information on microbial quality of edible crickets
thus the need for a profound insight into their safety. The objective of the study was to
determine the microbial load of two edible cricket species Scapsipedus marginatus and
Gryllus bimaculatus and to evaluate the efficiency of different processing methods
(boiling, sun-drying, freeze-drying, snap freezing and deep frying) in reducing microbial
counts. The wild-caught crickets were obtained from Nguruman (Kajiado County) and
Mbita (Homa Bay County) while the farmed crickets were reared at Animal Rearing and
Containment Unit at the International Centre of Insect Physiology and Ecology (icipe).
The cricket samples were screened for microbiota using culture-dependent method.
Fifteen adult crickets were pooled together and homogenized in Phosphate-buffered
Saline (PBS). The solution was used for mycological and bacterial isolation and analysis.
Bacterial microbiota was isolated using Nutrient agar (NA) and MacConkey agar (MCA).
Fungal microbiota was isolated using Sabouraud Dextrose Agar (SDA) and Potato
Dextrose Agar (PDA). The isolates were characterized morphologically and through
sequencing of bacterial 16S SSU rRNA genes and fungal internal transcribed spacer
(ITS) rRNA gene. Most of the bacterial isolates (70 %) on NA had characteristics typical
of members of Bacillus spp. while on MCA, most bacteria (90 %) had characteristics
suggestive of members of E. coli. Majority (80 %) of the fungal isolates on PDA had
characteristics similar to those of Aspergillus spp. while most isolates (80 %) on SDA
were members of Trichoderma spp. Microbial counts of fresh cricket samples were
generally high, with bacterial population ranging from 2.5 × 104 - 3.8 x 104 CFU /g fresh
weight. The fungal populations ranged from 2.0 × 104 – 3.0 × 104 CFU /g fresh weight.
Upon processing the microbial counts reduced considerably with bacterial counts ranging
from 0.1 x 102 – 0.8 x 102 CFU /g dry weight (boiled) and 0.7 x 103 – 2.2 x 103 CFU /g
dry weight (sun dried). The fungal counts ranged from 0.7 x 102 - 0.9 x 102 CFU /g dry
weight (boiled) and 0.9 x 103- 1.7 x103 CFU /g dry weight (sun-dried). Freeze-dried and
deep fried samples had no microbial counts. The diversity of bacteria and fungi species in
wild-caught crickets was considerably high compared to the lab-reared crickets with most
isolates belonging to species pathogenic to humans. Phylogenetic analysis revealed that
most bacterial isolates from the wild-caught crickets related to members of Bacillus spp.
(57 %), Staphylococcus spp. (43 %) and E. coli (14 %). Fungal isolates related to
Aspergillus spp. (57 %) and Penicillium roseopurpureum (29 %). From the farmed
crickets, most of the bacterial isolates related to E. coli (60 %), Enterobacter (40 %) and
Lactococcus garvieae (20 %). The fungal isolates related to Trichoderma asperellum (75
%), Aspergillus spp. (25 %) and Tetrapisispora fleetii (12.5 %). From the cricket samples
processed by boiling and sun drying, the microbial diversity was very low with only two
bacterial isolates related to Rickettsiella grylli (42 %) and Wolbachia spp. (29 %), while
the two fungal isolates related to members of Aspergillus spp. (67 %) and Trichoderma
asperellum (33 %). Thirteen potentially novel bacterial and fungal isolates from wild and
reared crickets had no close matches from gene bank and need further investigation. This
study shows that crickets harbor diverse microbial communities some of which are
potentially pathogenic. Deep-frying, freeze drying and snap freezing completely
eliminated bacterial and fungal contaminants thus minimizing microbial risks in crickets
meant for food.
Description
Thesis Submitted in Partial Fulfillment of the Requirements for the Award of the Degree of Master of Science (Microbiology) in the School of Pure and Applied Sciences of Kenyatta University, April 2019
Keywords
Microbial, Load And Diversity, Crickets (Gryllus Bimaculatus and Scapsipedus Marginatus), Protein, Food