Improved detection of coliforms and Escherichia coli in foods by a membrane filter method

Abstract

Analytical procedures based on filtration of homogenates through membrane filters, and particularly hydrophobic grid-membrane filters (HGMF), offer definite improvements in the enumeration of Escherichia coli and coliforms in foods. Whereas the counted specimen in pour plates may not usually be greater than 0.1 g, up to 1.0 g of ground beef, green beans, potato, cod, strawberries, or grapes could be filtered and counted on HGMF. Greatly improved limit of detection, reduced interference by noncoliforms, and complete removal of growth inhibitors such as polyphenols were demonstratedfor HGMF, using violet red bile and mFC agars. In addition, counting on HGMF eliminated a false-positive reaction caused by sucrose in ice cream.

Detection of Escherichia coli in Foods: Indole Staining Methods for Cellulosic and Polysulfone Membrane Filters

“…HGMF. Cellulose ester HGMF (47-mm diameter, 0.45-pm pore size, 1,984 grid cells) were printed in our laboratory from HAWP 04700 MF (Millipore Ltd., Mississauga, Ontario, Canada), using a cross-country ski wax as the hydrophobic material (6). Polysulfone HGMF (60 by 60 mm, 0.45-pm pore size, 2,500 grid cells) were purchased from QA Laboratories Ltd. (Etobicoke, Ontario, Canada).…”

“…For the ABP direct plating method, the procedure of Rayman et al (4) was used. Counts on HGMF were carried out as previously described (5,6). All filters were incubated on Tryptone bile agar for 24 h at 44.5°C and stained for indole by the appropriate procedure as described above.…”

“…We recently described a promising modification to this method, using hydrophobic gridmembrane filters (HGMF) (6). However, two problems arose when the new technique was used in other laboratories: (i) on cellulosic MF (Oxoid or Millipore), the above-mentioned problem caused difficulties in scoring HGMF which were densely packed with mixed flora; and (ii) on polysulfone MF (Gelman Tuffryn), indolepositive colonies did not stain.…”

An evaluation of fluorogenic and chromogenic assays for the direct enumeration of Escherichia coli

“…These involve long incubation periods and several steps thus making it time consuming. Filtration and direct plating techniques on to selective media (Anderson & Baird Parker 1975;Sharpe et al 1979) are also used and accepted, although inhibitors in some media may be lethal to damaged E. coli cells. It is for these reasons that rapid methods have been investigated.…”

Membrane filtration of dairy products for microbiological analysis

“…The filterability problem has remained to the present day, such that the 1978 edition of Standard Methods for the Examination of Dairy Products (7) contains only one use for MF-that of the somatic cell count in raw milk. Since Sharpe and co-workers (11)(12)(13)(14)(15)(16) have recently shown the attractiveness in food microbiology of MF, and particularly hydrophobic grid MF techniques, a solution to the problem of dairy product filterability was sought. Use of a 4% protease-2.2% Triton X-100 incubation to improve membrane filtration of raw milk for somatic cell counts was recently described by Cousins et al (5), but these conditions may be too severe for viable cell counts.…”