Comparison of Methods of Estimating the Number of Escherichia coli in Edible Mussels and the Relationship between the Presence of Salmonellae and E. coli

Abstract

SUMMARY: Of 4 methods of assessing the bacteriological quality of mussels the semiquantitative method of the Fishmongers’Company was the most useful from the public health aspect where facilities may be limited. A modification of Clegg & Sherwood's method, using pour plates, was more sensitive and more simple to perform than the original method and is to be recommended for routine use. Two Escherichia coli cells/ml of mussel tissue probably represents the upper acceptable limit for satisfactory grading of mussels. The presence of small numbers of salmonellae in purified mussels is probably not highly significant but tests for their presence will yield useful epidemiological data.

Indicator bacteria in freshwater and marine molluscs

“…completely immersed in a water bath controlled at 44 ± 0 .25 ° C yielded the E . coli counts after 24 h (Thomas & Jones, 1971) . Total viable numbers (TVC) were obtained from nutrient agar (Oxoid CM 3) with separate incubation at 37 ° C for 24 h and at 25 ° C for 48 h .…”

“…These workers also reported on the presence of Escherichia coli and other indicator organisms as did Thomas & Jones (1971) who concluded that the incidence of salmonellae in their unpurified mussels was related to the incidence of salmonellosis in the Conwy population. Ayres (1975) compared the numbers of E. coli, total coliforms at 37 ° C, faecal streptococci and Clostridium perfringens (welchii) from oysters, mussels and hard clams and considered that the latter three bacterial tests have a complementary value but do not replace E. coli as Hydrobiologia 111, 93-102 (1984) .…”

“…The shells were cleansed with a small scrubbing brush under running water, dried with absorbant paper, and the animals opened using a sterile scalpel and appropriate twisting pressure (Thomas & Jones, 1971) . The shell water was discarded (Trollope, 1982) .…”

A rapid and simple method for the detection and enumeration ofEscherichia coliin cleansed shellfish

“…3) pour plates (Thomas & Jones, 1971) in parallel with a two-stage 5-tube MPN technique, using Formate Lactose Glutamate Medium (FLG) as the initial medium. With the former method, which has a detection limit of 300 E. coli/ 100 ml, one plate was prepared per sample using 1 ml of the 1:3 dilution.…”

A tentative national reference procedure for isolation and enumeration of Escherichia coli from bivalve molluscan shellfish by most probable number method

“…There is no single national reference method in the UK for enumerating E. coli in live bivalve shellfish, despite widespread use of a guideline upper limit of 230 E. coli/100 g in shellfish on the wholesale market . A variety of methods are used, including liquid broth most probable number (MPN) procedures and molten agar, roll tube and pour plate techniques (Thomas & Jones 1971 ;Collins & Lyne 1984;Trollope 1984;Anon. 1986).…”