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Looking for Foodborne Germs and Their Resistance to Antibiotics: Ground Beef

Updated August 21, 2019
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Russ Daly

Professor, SDSU Extension Veterinarian, State Public Health Veterinarian

Written collaboratively by Russ Daly, Alan Erickson, Laura Ruesch, Zachary Lau, and Deb Murray.

South Dakota State University is in its third year of participation in the National Antimicrobial Resistance Monitoring System (NARMS), with their Food Safety Microbiology (SD-FSM) lab (part of the Animal Disease Research and Diagnostic Laboratory in Brookings) working with the retail meat portion of the program. The SD-FSM tests fresh chicken, ground turkey, ground beef, and pork chops purchased in grocery stores in North and South Dakota for the presence of certain germs. The germs identified are then further tested for resistance to common antibiotics that might be used in treating human illness.

The SD-FSM has recently compiled NARMS results for the period from June 2018 through May 2019. This report summarizes the results for ground beef. Separate reports will describe the results from poultry products and pork products. The NARMS program looks for 3 different germs in ground beef. Salmonella is a significant cause of foodborne illness in the US, while the other 2 (generic E. coli and Enterococcus, found more routinely in meat products) are examined as indicator organisms for antibiotic resistance.

Prevalence of Bacteria in Ground Beef From North and South Dakota Grocery Stores

During the investigation period (June 2018 – May 2019), Salmonella was only found once in the 120 (0.8%) grocery store samples tested (Table 1). For E. coli, 53.3% (32/60) of the samples were positive, while 83.3% (50/60) contained Enterococcus – both increases over 2017-18. Results were similar between samples taken from North Dakota and those from South Dakota for E. coli and Enterococcus prevalence.

Since national statistics for the same time period aren’t yet available, it’s not possible to directly compare the Dakotas data with national data. However, information from 2015 – the most recent year available – showed that Salmonella levels were also quite low nationally (0.4%). Levels for the indicator bacteria E. coli and Enterococcus found in ground from stores in the Dakotas were lower than the 2015 national averages, which were 47.3% for E. coli and 87.7% for Enterococcus.

Table 1. Prevalence of bacteria in ground beef from North and South Dakota grocery stores, June 2018-May 2019.
 
North Dakota
South Dakota
Dakotas Total
 
Bacteria
No. Tested
Pos.
%
No. Tested
Pos.
%
No. Tested
Pos.
%
2017-18 rate
2015 National % Pos.
Salmonella
62
0
0
58
1
1.7%
120
1
0.8%
0
0.4%
E. coli
30
15
50.0%
30
17
56.7%
60
32
53.3%
41.7%
47.3%
Enterococcus
30
24
80.0%
30
26
86.7%
60
50
83.3%
73.3%
87.7%

Antimicrobial Resistance in Bacteria From Ground Beef From Stores in North and South Dakota

Isolates (individual growths) of the 3 germ species were submitted to the FDA for antimicrobial resistance testing. Data isn’t available yet for the June 2018-May 2019 time period; however, the resistance of germs found during the period of February, 2017 through December 2017 is summarized below (Table 2).

During this time period, none of the ground beef samples tested positive for Salmonella.

For the indicator organisms E. coli and Enterococcus, the action of 14 and 16 antibiotics respectively, is measured. A germ is classified as “Multi-Drug Resistant” (MDR) if it is resistant to 3 or more different classes of antimicrobials. During the February-December 2017 time period, 36 E. coli isolates were obtained from ground beef, with 3 of them (8.3%) being MDR. The great majority (27/36) of these isolates were susceptible to all antimicrobials they were tested against; tetracycline (25% of isolates resistant) was the drug with the most resistance.

Thirty-five Enterococcus isolates were obtained from ground beef during the same time period, with 10 of them (28.6%) MDR isolates. Antibiotic resistance of these gram-positive isolates was most commonly toward lincomycin, with 91.4% of isolates resistant.

Whole genome sequencing methods employed at FDA during this project enables the detection of genes that code for antibiotic resistance in the bacterial genome. All Salmonella and Campylobacter, and half the E. coli isolates are submitted for WGS. Five of 18 E. coli isolates contained detectable resistance genes. These detections correlated well with the actual antibiotic resistance demonstrated through conventional lab methods.

Table 2. Antibiotic resistance in bacterial isolates from ground beef from North and South Dakota grocery stores, February-December 2017.
   
# of antibiotics that isolates were resistant to
 
Bacteria
Total
0
1
2
3
4
5
# of MDR isolates
Salmonella
0
-
-
-
-
-
-
-
E. coli
36
27
5
1
1
1
1
3/36
Enterococcus
35
1
3
19
7
1
4
12/35

Summary

South Dakota State University’s work with NARMS continues to produce information about the prevalence of potentially illness-causing germs in ground beef and other meat products in North and South Dakota. This year’s data indicates that Salmonella continues to be extremely uncommon in ground beef purchased in the Dakotas. Isolation rates of these bacteria in ground beef were slightly higher than those found in 2017-18. Comparing this data with contemporary national data (once it’s available) will provide even better information regarding food safety in the Dakotas, and work in future years will help detect trends in isolation rates over time.

The project is also measuring levels of antibiotic resistance in those germs as well as certain indicator germs. As changes occur in the use of antibiotics in beef production, monitoring germ resistance to antibiotics will become important to determine the effect, if any, of shifts and declines in antibiotic use in beef production. As the project continues, trends in antibiotic resistance over time will be detectable.