Recirculating immunomagnetic separation and optimal enrichment conditions for enhanced detection and recovery of low levels of Escherichia coli O157:H7 from fresh leafy produce and surface water.
The objective of this study was to develop a rapid, simple method for enhanced detection and isolation of low levels of Escherichia coli O157:H7 from leafy produce and surface water using recirculating immunomagnetic separation (RIMS) coupled with real-time PCR and a standard culture method. The optimal enrichment conditions for the method also were determined. Analysis of real-time PCR data (C(T) values) suggested that incubation of lettuce and spinach leaves rather than rinsates provides better enrichment of E. coli O157:H7. Enrichment of lettuce or spinach leaves at 42 degrees C for 5 h provided better detection than enrichment at 37 degrees C. Extended incubation of surface water for 20 h at 42 degrees C did not improve the detection. The optimized enrichment conditions were also employed with modified Moore swabs, which were used to sample flowing water sites. Positive isolation rates and real-time PCR results indicated an increased recovery of E. coli O157:H7 from all samples following the application of RIMS. Under these conditions, the method provided detection and/or isolation of E. coli O157:H7 at levels as low as 0.07 CFU/g of lettuce, 0.1 CFU/g of spinach, 6 CFU/100 ml of surface water, and 9 CFU per modified Moore swab. During a 6-month field study, modified Moore swabs yielded high isolation rates when deployed in natural watershed sites. The method used in this study was effective for monitoring E. coli O157:H7 in the farm environment, during postharvest processing, and in foodborne outbreak investigations.
Himathongkham
Sunee
S
Food and Drug Laboratory Branch, California Department of Public Health, Richmond, California 94804, USA. sunee.himathongkham@cdph.ca.gov
Dodd
Mary Lee
ML
Yee
Jenny K
JK
Lau
David K
DK
Bryant
Raymond G
RG
Badoiu
Alexandru S
AS
Lau
Henry K
HK
Guthertz
Linda S
LS
Crawford-Miksza
Leta
L
Soliman
Mary A
MA
D001422:Bacterial Adhesion / Q000502:physiology; D015169:Colony Count, Microbial; D003257:Consumer Product Safety; D004783:Environmental Microbiology; D019453:Escherichia coli O157 / Q000302:isolation & purification* / Q000502:physiology; D005516:Food Microbiology; D018189:Immunomagnetic Separation / Q000379:methods*; D018545:Lactuca / Q000382:microbiology*; D016133:Polymerase Chain Reaction / Q000379:methods; D018724:Spinacia oleracea / Q000382:microbiology*; D013696:Temperature; D013997:Time Factors; D014871:Water Microbiology