BRITISH SOCIETY FOR MICROBIAL TECHNOLOGY

SCIENTIFIC WORKSHOPEnterics – a bog standard approach?

J.D. Perry

Freeman Hospital

Newcastle upon Tyne

An Appraisal of Enteric Culture Media

27 November 2009

In enteric microbiology, culture media may be used for isolation of:

• Salmonella species.

• Shigella species.

• E. coli O157

• Campylobacter species

• Vibrio cholerae.

• Yersinia enterocolitica

• Clostridium difficile

Conventional media commonly used in clinical microbiology for

Salmonella/Shigella isolation.

• Deoxycholate citrate agar

• XLD agar

• Salmonella-Shigella medium.

• Hektoen Enteric agar.

• MLCB agar (Salmonella only)

Advantages of conventional media:

• Relatively inexpensive and effective.

• May allow for isolation of other pathogens e.g.. Shigella.

Disadvantages of conventional media:

• Often highly non-specific and labour intensive.

Examples of Chromogenic agars for isolation of Salmonella.

• Rambach medium (Chromagar Co.)

• SMID agar (bioMérieux)

• CSE Agar (PPR Diagnostics)

• Compass Salmonella agar (Biokar Diagnostics).

• Salmonella Chromogenic agar (Oxoid).

• Chromagar Salmonella (Chromagar Co.)

• ABC medium (Lab M).

Rambach agar (Chromagar Co.)

Substrates used:

5-Bromo-4-chloro-3-indolyl-ß-D-galactoside

(X-Gal):

E. coli, Klebsiella, Enterobacter, Citrobacter.

Propylene glycol: Salmonella

[Rambach, A. 1990, Appl. Environ. Microbiol. 56:301-303].

SM-ID agar (bioMérieux)

Substrates used:

5-Bromo-4-chloro-3-indolyl-ß-D-galactoside

(X-Gal):

E. coli, Klebsiella, Enterobacter, Citrobacter.

Sodium Glucuronate: Salmonella.

[Poupart, M.C., et al. ECCMID 1991, abstr. 1254].

5-bromo-6-chloro-3-indolyl-octanoate (magenta-caprylate)

HNBr

ClO-octanoic acid

Esterase production by Salmonella using magenta-caprylate.

A range of chromogenic media

is available for detection of

Salmonella spp based on esterase

detection.

ASAP (AES CHEMUNEX) for isolation of Salmonella from stools.

Salmonella species producing C8 esterase

‘Coliform’ producing ß-glucosidase.

Further examples of Salmonella detection using an esterase substrate.

BBL™ CHROMagar™ SalmonellaSM-ID 2 for Salmonella

ABC medium (Lab M).

3,4-cyclohexenoesculetin -ß-D-galactoside:

E. coli, Klebsiella, Enterobacter, Citrobacter.

5-bromo-4-chloro-3-indolyl-alpha-D-galactoside:

Salmonella (green colonies).

[J. Clin. Microbiol. 1999; 37: 766-768].

Salmonella napoli isolated from a stool sample on ABC medium.

SalmonellaSalmonella

(α-GAL +)(α-GAL +)E. coliE. coli

(α and ß GAL +)(α and ß GAL +)

Advantages of chromogenic media:

• Highly specific for target pathogens and therefore less labour intensive.

Disadvantages of chromogenic media: • Relatively expensive.

• Do not usually allow for isolation of more than one target pathogen.

• May be less sensitive than conventional media.

Comparative evaluation of four chromogenic media with Hektoen enteric agar using 916 stool specimens

from three hospitals.[Perez et al. 2003. JCM 41:1130-1134.]

ABC medium

COMPASS agar

CHROMagar Salmonella SM-ID Hektoen

No. of Salmonella 57 60 57 55 63

No. of false positives 17 108 130 83 215

Evaluation and Implementation of a Chromogenic Agar Medium for Salmonella Detection in Stool in Routine

Laboratory Diagnostics.[Journal of Clinical Microbiology, February 2009, p. 456-458, Vol. 47, No. 2]

Sensitivities and specificities of the tested media calculated from culture results after 48 h and including enrichment

Medium No. of Salmonella.Sensitivity

(%)No. of false-

positivesSpecificity (%)

SS 23 71.9 277 78.8

XLD 31 96.9 261 80

Hektoen 27 84.4 245 81.3

BBL 28 87.5 56 95.7

SM-ID 2 25 78.1 51 96.1

Q: How good are chromogenic media at inhibiting the growth of

commensal bacteria ?

A: Most media for Salmonella allow the growth of most

Enterobacteriaceae.

Selectivity of various Salmonella agars

No. of strains Hektoen XLD ABC Chromagar Rambach SMIDSalmonella spp. 63 95.2 98.4 100 100 96.8 100Total non-Salmonellae 372 70.4 90.9 87.9 77.4 79.8 96.5Total ß-gal producers 235 73.6 89.8 95.7 93.6 98.7 99.6Total E.coli 108 45.4 78.7 96.3 95.4 98.1 100

Stool sample containing Salmonella on Rambach agar

Stool sample containing Salmonella on SM-ID agar.

Stool sample containing Salmonella on ABC medium.

Stool sample containing Salmonella on XLD medium.

A new approach to the selective isolation of Salmonella using

suicide substrates.

Principle of ‘suicide substrate’

• A non-inhibitory substrate which is cleaved enzymatically to release an inhibitory product.

• Resistance may be due to:• Resistance of the target site to the inhibitory

agent.• Lack of an appropriate enzyme for cleavage

of the substrate.• Reduced uptake of the substrate by the

bacterial cell.

Identification of a novel selective agent: L-alanyl-1-aminoethylphosphonic acid or alafosfalin.

Fig 1: Structure of alafosfalin:

H N

CH CH

CC C

O

2

33

N

HH

O

P OH

OHH

Composition of medium for alafosfalin MIC’s:

• 16 amino acids.

• Buffered salt solution.

• Purine/Pyrimidine bases

• Agar/Water

• No proteins/peptones

Average MIC results of alafosfalin for some common

faecal organisms.

• Salmonella spp. (n=63) 10.2 mg/l

• E.coli (n=108) 0.7 mg/l

• Citrobacter spp. (n=28) 2.8 mg/l

• Enterobacter spp. (n=31) 6.5 mg/l

• Hafnia alvei (n=10) 2.8 mg/l

• Klebsiella spp. (n=34) 6 mg/l

• Serratia spp. (n=13) 3.2 mg/l

Genera resistant to alafosfalin (MIC >32 mg/l)

• Morganella spp.

• Proteus spp.

• Providencia spp.

• Pseudomonas spp.

• Acinetobacter spp.

Composition of ‘Modified’ ABC medium

• 16 amino acids.

• Buffered salt solution.

• Purine/Pyrimidine bases

• Agar/Water

• Alafosfalin (1 mg/l)

• Chromogenic mix

• 0.5 % Sodium deoxycholate

Selectivity of various Salmonella agars

% strains recovered

No. of strains Hektoen ABC Mod-ABC

Salmonella 63 95.2 100 98.4

Non-Salmonellae 372 70.4 87.9 54.6

ß-GAL positives 235 73.6 95.7 46.0

E.coli 108 45.4 96.3 10.2

Field trial study 1: Newcastle, UK.

• 797 consecutive liquid stool samples from Newcastle PHL were cultured onto:

• Hektoen enteric, ABC medium, ‘Modified’ ABC medium.

• Selenite broth enrichment followed by: Hektoen enteric, ABC medium, ‘Modified’ ABC medium.

Field trial 1 results:

Total Salmonella isolated: 33

Direct culture:

Hektoen enteric agar: 18 (54.6 %)

ABC medium: 17 (51.5 %)

‘Modified’ ABC: 25 (75.8 %)

Post enrichment:

Hektoen enteric agar: 32 (97 %)

ABC medium: 33 (100 %)

‘Modified’ ABC: 33 (100 %)

Field trial 1: results

False positives on direct culture and

post-enrichment:

Hektoen enteric agar: 172 (21.6 %)

ABC medium: 15 (1.9 %)

‘Modified’ ABC: 4 (0.5 %)

Stool sample containing Salmonella on Rambach agar

Stool sample containing Salmonella on SM-ID agar.

Stool sample containing Salmonella on ABC medium.

Stool sample containing Salmonella on Hektoen Enteric agar

Stool sample containing Salmonella on XLD medium.

Stool sample containing Salmonella on Modified ABC medium (containing alafosfalin).

Stool sample containing Salmonella on Rambach agar.

Stool sample containing Salmonella on SM-ID medium.

Stool sample containing Salmonella on ABC medium.

Stool sample containing Salmonella on Hektoen Enteric agar.

Stool sample containing Salmonella on XLD medium.

Stool sample containing Salmonella on Modified ABC medium (containing alafosfalin).

Conclusions on the use of the ‘suicide substrate’ alafosfalin as a selective agent.

Advantages

• Cost of inhibitor (1 mg/l): 10 p per litre of agar.

• Highly stable agent.

• Improves selectivity and specificity.

Disadvantages

• Inhibits Salmonella typhi. (or use di-alanyl fosfalin).

• Of limited use post-enrichment.

• Specialised medium is required.

OSCM II using InhibigenTM technology.

[www.oxoid.com]

Conclusions on use of Salmonella media:

• Chromogenic media offer a means of detecting Salmonella with high specificity and reduced labour time when compared with conventional agars.

• There is a lack of evidence that they have any superiority for detection of Salmonella when compared with conventional agars e.g. XLD / Hektoen and some studies suggest a lack of sensitivity.

• Large evaluation studies are lacking – particularly with some newer media.

• Suicide substrates or InhibigensTM show promise for improved detection of Salmonella but further studies with clinical samples are needed.

Media for Shigella:

• Examples include Hektoen enteric agar, DCA, SS agar, XLD etc and all are based on similar principles e.g. non fermentation of lactose / sucrose.

• Comparative studies are lacking.

• No specific chromogenic media are commercially available.

SALSA (AES CHEMUNEX)

ASAP –

Chromogenic Salmonella agar.

XLD –

For Salmonella and Shigella detection.

A range of media is available for isolation of Campylobacter.

• Traditional media were blood-based and employed a cocktail of antimicrobials including polymyxin, vancomycin, trmethoprim, cephalothin & amphotericin.

• Charcoal was shown to be a useful substitute for blood (Bolton et al. JCM; 19; 169-71).

• Superior selective agents have been identified e.g. cefoperazone.

• Modified charcoal cefoperazone deoxycholate agar (CCDA) is widely used.

• Preferred temperature of incubation is 42°C under microaerophilic conditions (typically 5% oxygen / 10% carbon dioxide and 85% nitrogen).

• No single medium and no single set of incubation conditions will isolate all Campylobacter.

Chromogenic media for Campylobacter

• These have been developed for enumeration of Campylobacter in food and there are no journal reports describing evaluation with clinical samples.

Stool sample on Campylosel

Same stool sample on CCDA

Same stool sample on Campy Food ID

CASA (AES CHEMUNEX)

Brilliance Campycount Agar(Oxoid)

Media for E. coli O157:H7

• Media are designed on the principles that this serotype differs from most E. coli in that:

• Most strains of E. coli O157 fail to generate acid from sorbitol.

• Most strains of E. coli O157 fail to produce ß-glucuronidase.

• Other serotypes generating verotoxin will generally not be detected on media designed for E. coli O157.

Media for E. coli O157:H7

• Sorbitol MacConkey is the most commonly used selective medium.

• Tellurite and cefixime are commonly used for increased selectivity.

• Specificity remains limited as some species such as E. hermannii and Proteus species occasionally grow and produce non-fermenting colonies.

CHROMagar E. coli O157

BBL™ CHROMagar™ O157

Rapid and sensitive detection of Shiga toxin-producing Escherichia coli from non-enriched stool specimens by real-time PCR in

comparison to enzyme immunoassay and culture.

[J Clin Microbiol. 2009 Jul;47(7):2008-12.]

• Used BBL™ CHROMagar™ O157 and EIA to validate a PCR method but the study was not designed for evaluation of the medium.

• Chromogenic media for E. coli O157 show good specificity when pure strains are tested but evaluations with clinical samples are lacking.

Media for C. difficile:

A) CLO (bioMerieux)

B) Oxoid

C) BBL

D) CCEY (Bioconnections)

E) CCEY plus lysozyme

A B C

D E

Transparency Declaration:

In the last ten years, the presenter has received financial support for research or consultancyfrom suppliers of chromogenic culture media including bioMérieux, Becton Dickinson, Lab M and Bio-Rad.