Bacteriolation Lab Report

Detection of listeria spp

Contamination of food and dairy products with Listeria is the major cause of foodborne disease in human. Since researchers have found out that listeria is a foodborne pathogen, there is a continuous challenge on isolation of bacteria from food and other samples [115]. The primary studies indicated that Listeria is able to grow at low temperature so researchers used this phenomenon for isolation of bacteria from clinical samples by culturing for a long time in 4oC but this method could not differentiate the injured listeria cells which cannot survive and grow at stress condition[116]. Using time consuming enrichment step and lack of differentiation of damaged listeria should be taken into consideration, as the results

Two of most important reference methods for detection of Listeria in all food samples are: FDA bacteriological and analytical methods (BAM) and International Organization of Standard (ISO) 11290 methods [123]. In FDA BAM methods enrichment carried out in media containing selective bacteriostatic agent (nalidixic acid and acriflavine) along with cycloheximide as antifungal agent. The temperature for enrichment is 30oC for 48hrs. The ISO 11290 is two-step process: first enrichment in half Fraser broth for 24hrs then transferred to full Fraser media. The Fraser media contains the same bacteriostatic agent as in FDA BAM method and also contains esculin for detection of ẞ-D- galactosidase activity of Listeria. However using bacteriostatic agent can have a reverse effect on bacterial selection of which attributes to damage of bacteria in stress condition. To avoid this effect in FDA BAM, the agent added after 4hrs of incubation to allow the injured cells to recover and grow in media, but in ISO 11290 at first step of enrichment, the half concentration of agent is added. In both media the buffering capacity of media is very high and leads to enhanced cell growth and

Using chromogenic media which is based on essential pathogenic virulence factors of listeria has been administered recently [129]. Detection of Listeria monocytogenes in Listeria agar with Ottaviani and Agosti(ALOA) is based on detection of phosphatidylinositol-specific phospholipase C(PI-PLC) activity which Listeria monocytogene and some strains of L.inanovii hydrolysis the l-α-phosphatidylinositol and produce a fatty acid and form a opaque halo around the colonies [130, 131]. The other media which are similar to ALOA plate include: BCM listeria (Biosynth, Switzerland), OCLA (Oxoid, UK), CHROMagar_ Listeria (BD e Diagnostic Systems, USA) and OAA (bioMérieux, France). Rapid L’mono_ (Bio-Rad Laboratories, USA), could distinguish between hemolytic and non hemolytic listeria based on fermentation of xylose. L.ivanovii could ferment the xylose and produce blue colonies with yellow halo but L.monocytogene is non hemolytic and could not ferment the xylose and produce blue colonies without a halo. Other species of Listeria could not cleavage the 5-bromo-4-chloro-3-indolyl-myo-inositol-1-phospjhate (X-IP), a substrate of PI-PLC and grow with white colonies with or without a yellow halo [132]. The CHROM agar could be