Microbiological quality of ready-to-eat lettuce, lactuca sativa, sold in supermarkets and restaurants in Metro Manila

Date of Publication


Document Type

Bachelor's Thesis

Degree Name

Bachelor of Science in Biology

Subject Categories



College of Science

Thesis Adviser

Juliene Co


Raw salad vegetables are available in ready-to-eat (RTE) packages and in self-serve salad bars. However, despite the health benefits this food offers, the microbial quality of these products is not regularly evaluated. An assessment of RTE salad vegetables was done to determine microbial load using total coliform counts. Seven samples were obtained from various supermarkets and restaurants around Metro Manila, consisting of3 packaged and 4 non-packaged lettuce samples. Twenty-five grams of each sample was suspended in 100 mL of sterile distilled water and placed in a shaker for 30 minutes. A 10-fold serial dilution of the suspension was performed, followed by spread plate technique on MacConkey agar done in triplicate. Total coliform counts showed ≥104 CFU/g for all samples, which is considered unsatisfactory based on Consumer Report(2010) industry standards. However, no criteria on acceptable levels of total coliforms in raw vegetables werefound in the Public Health Laboratory Service (PHLS) guidelines.Total gram-negative counts were also compared between samples to assess potential shelf life. Triple-washed packaged lettuce yielded significantly lower total gram-negative counts than single pre-washed (p=0.0073) and washed (p=0.0398) packaged samples. Surprisingly, single-washed packaged samples had significantly higher microbial counts than mostnon-packaged samples. Representative E. coli-like colonies from each collection sample were also isolated, purified, and identified through IMViC, API 20® E test, or 16S rDNA sequencing. All of the E. coli-like colonies turned out to be non-E.colibacteria and mostly environmental coliforms, although some have been documented to associate with clinical disease in immunocompromised patients. These 17 E. coli-like isolates purified were identified as belonging to Enterobactersp.(6),Klebsiellasp.(2),Kluyverasp. (4),Rahnellaaquatitis(1), Raoultellasp.(2),and Aeromonassp.(2). This study showed that although total coliform counts of ≥104 CFU/g are considered unsatisfactory according to Consumer Reports standards, the coliforms identified were primarily environmentally derived. This suggests that usingE. coli (rather than total coliforms) as an indicator of fecal contamination may be more appropriate for raw vegetables, in agreement with PHLS recommendations. Among packaged samples, triple-washed bagged lettuce may have a longer shelf life than single-washed bagged samples, owing to its significantly lower microbial load. A more thorough microbial assessment of fresh produce using E. coli as an indicator organism should be considered in order to evaluate the microbial quality of raw vegetables sold to consumers.

Abstract Format






Accession Number


Shelf Location

Archives, The Learning Commons, 12F, Henry Sy Sr. Hall

Physical Description

1 computer disc ; 4 3/4 in.


Salad vegetables—Microbiology

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