Evaluation of three methods using deoxyribonuclease production as a screening test forSerratia marcescens

Abstract

SYNOPSIS Deoxyribonuclease (DNase) produced by Serratia marcescens is a characteristic feature which is useful in distinguishing this organism from closely related members of the Enterobacteriaceae. In an assessment of three methods of perf"orming the DNase test as a screening procedure for Serratia marcescens, the conclusion was reached that the standard test was most suitable for use in the clinical bacteriology laboratory.

Search for Serratia

“…In these cases a few simple confirmatory tests sufficed for the purposes of this study. As a further safeguard against missing any Serratia strains during this period, another 276 consecutive organisms which were lactose-fermenters on the MacConkey medium were tested for deoxyribonuclease (DNase) production since this is a characteristic of Serratia marcescens (Black, Hodgson, and McKechnie, 1971) and any strains giving a positive or weakly positive result were fully investigated biochemically by the tests shown in Table I The antibiotic sensitivities of the Serratia marcescens isolates were tested on Columbia agar (Oxoid CM 331) using Mast sensitivity discs (Mast Laboratories, Liverpool). The technique used was that of Stokes (1968) which involves the direct comparison on each plate of the test strain and a control organism (Escherichia coli NCTC 10418) of known sensitivity.…”

Hydrolysis of casein: a differential aid for the indentification ofSerratia marcescens

“…This organism has demonstrated an easily recognizable and reproducible capability to hydrolyze casein in a defatted milk medium. Serratia marcescens has become a significant organism (Davis, Foltz, and Blakemore, 1970;Sanders, Luby, Johanson, Barnett, and Sanford, 1970;Black, Hodgson, and McKechnie, 1971) in our nosocomial infections and hospital epidemiology. Utilizing the work of Brown and Scott Foster (1970), we have been using a defatted milk agar (Hasting, 1903;Gordon and Smith, 1955;Ajello et al, 1963) in our routine as a diagnostic aid in the identification ofPseudomonas aeruginosa by its hydrolysis of casein and pigmentation.…”

“…Utilizing the work of Brown and Scott Foster (1970), we have been using a defatted milk agar (Hasting, 1903;Gordon and Smith, 1955;Ajello et al, 1963) in our routine as a diagnostic aid in the identification ofPseudomonas aeruginosa by its hydrolysis of casein and pigmentation. Preliminary examination of these milk agar plates reveals a nonpigmented colony with a recognizable zone of hydrolysis that is distinctly different than Pseudomonas and is confirmed to be Serratia marcescens by its characteristic DNase production (Elston and Elston, 1968;Black et al, 1971) decarboxylase activity (Ewing, Davis, and Edwards, 1960), sensitivity profiling (Bauer, Kirby, Sherris, and Turck, 1966), and colony characteristics on other Received for publication 3 October 1972. media. This reproducible colony characteristic has successfully withstood comparison against 438 strains of the families, Enterobacteriaceae and Pseudomonadaceae.…”