Systematic Veterinary Bacteriology and Mycology

MORPHOLOGY AND CULTURAL CHARACTERISTICS

Morphology

• L.monocytogenes are medium sized, Gram+ve rods, non-spore forming and non-acid fast.
• Old cultures stain Gram –ve. From rapidly growing cultures or animal tissues the cells can appear coccal.
• They are motile by few (1-5) peritrichous flagella. They are motile at room temperature, but not at 37°C.

Cultural characteristics

• L.monocytogenes is able to grow at temperature ranges from 4 to 45°C and grow at pH range of 5.5 to 9.6.
• It is relatively resistant to high salt (10%) concentrations. They are facultative anaerobes.
• The growth is enhanced by agar enriched with glucose, blood, liver extract and by 10% Co2.
• They grow on nutrient agar, blood agar but not on MacConkey agar.
• Small transparent colonies with smooth borders appear on blood agar in 24hrs, becoming grayish white in 48hrs.
• L.monocytogenes and other non-pathogenic listeria produce narrow zones of beta haemolysis, often only under the colony itself.
• L.ivanovii produces a comparatively wide zone of haemolysis and is very similar in appearance to beta haemolytic streptococci.
• L.monocytogenes produces a CAMP reaction with the haemolysis of S.aureus.
• In contrast, Listeria ivanovii is negative in the CAMP reaction with S.aureus.
• On TSA or BHI agar, these colonies have a characteristic blue-green sheen when light is reflected obliquely at a 45° angle off their surface.
• In fluid medium, slimy tenacious precipitate forms after incubation for several days.
• L.monocytogenes, particularly shows the characteristic tumbling motility when a 2-4 hr broth culture, incubated at 25°C, is examined by the hanging drop method.
• This motility is an end-over-end tumbling of individual cells with periods of quiescence.
• When grown in semisolid motility media the Listeria spp. give an unusual umbrella shaped growth in the subsurface.