Listeria on Blood Agar: Introduction, Morphology, Culture Characteristics, Pathogenesis, Lab Diagnosis and Treatment

Listeria on blood agar and its colony morphology

Introduction of Listeria 

Listeria monocytogenes on blood agar and its colony morphology as shown above image. It is a Gram-positive bacterium that causes a serious infection called   Listeriosis and it is caused by eating food contaminated with the bacterium Listeria Monocytogenes. It is a rod-shaped, intracellular aerobic pathogenic bacterium that invades the cytoplasm of living cells. It develops a distinctive rocket tail structure to help push through the cytoplasm. Eventually, these “rockets” push bacteria into neighboring cells, propagating the infection and it has recently been recognized as an important public health problem in the United States that produces but also infections worldwide. An important cause of infections in animals and men. The disease affects primarily persons of advanced age, pregnant women, new-born, and adults with weakened immune systems. However, persons without these risk factors can also rarely be affected.

Classification of Organism

  • Domain: Bacteria
  • Division: Firmicutes
  • Class: Bacilli
  • Order: Bacillales
  • Family: Listeriaceae
  • Genus: Listeria
  • Species:
    L. monocytogenes
    L. dentrificans
    L. grayi
    L. innocua
    L. ivanovii
    L. seeligeri
    L. murayi
    L. welshimeri

Morphology of Listeria

The morphology of the genus Listeria is small, Gram-positive coccobacilli or rod-shaped having a size of 1-3µmX 0.5 µm. They are motile at 25°C showings tumbling motility but are non-motile at37°C. They produce peritrichous flagella optimally at 20-30°C, but they can not produce flagella at all. They are non-sporing, non-capsulated, and acid-fast. They may resemble the morphology of diphtheroid that makes the Microbiologists identify from Diptheriods, which are mistaken and specimens are discarded.

Culture characteristics of Listeria

L. monocytogenes is an aerobe and facultative anaerobe. It has an unusual feature having the capability to grow at refrigeration temperature. The temperature range for growth is 3-45°C. But optimal temperature for growth is 30°C. In blood agar, it produces small, grey, translucent drop-like colonies surrounded by a small zone of indistinct beta hemolysis like streptococci but it is catalase test positive. It may require incubation up to 48 hours to produce visible growth. Gram stain is very helpful for checking the morphology of colonies.

In Muller-Hinton agar (MHA), colonies appear pale blue-green when viewed from the side ( 45° angle) with a beam of white light.

Biochemical tests of Listeria

Catalase test: Positive

Indole, oxidase, and urea hydrolyzation test: Negative

It ferments glucose and maltose with acid production.

CAMP test: Positive.

Listeria monocytogenes inoculated at right angles to b-hemolytic Staphylococcus aureus. Note the arrow-shaped zone of weakly enhanced hemolysis indicating a positive CAMP test.

Pathogenesis of Listeria

Listeria Monocytogenes enter through the Gastrointestinal tract after infections of contaminated foods such as cheese or vegetables. The cell wall surface protein called Interanalin interacts with E –cadherin and enters into epithelial cells. Bacteria produce listeriolysin. It can move from cell to without being exposed to antibodies, complement, polymorphonuclear cells.

Source of Infection

Listeria monocytogenes can be found in a variety of dairy products, vegetables, fish, and meat products. Listeria monocytogenes, unlike most other harmful bacteria, will grow slowly on foods stored in a refrigerator. This organism can also be spread by contact with an infected product or surface, such as hands or countertops, during food preparation.

Prone to Listeriosis are-

  • Pregnant women
  • New-born
  • People with weakened immune systems
  • People who are taking immuno-suppressing medication.

Clinical features

Following are the clinical features of Listeriosis-

  • Vomiting
  • Nausea
  • Cramps
  • Diarrhea
  • Severe Headache
  • Constipation and
  • Persistent fever.

Neonatal Infections

New-born rather than pregnant women themselves suffer the serious effects of infection in pregnancy.

Adult Infections

Adults may present with bacteremia. Meningoencephalitis occurs most commonly in Immuno-suppressed patients in whom Listeria is one of the more common causes of Meningitis. The disease can be insidious to fulminant.

 Listeriosis Presenting with Meningitis

Immunocompromised adults are at risk for a serious infection of the bloodstream and central nervous system (brain and spinal cord). Meningitis occurs in about half of the cases of adult Listeriosis. Symptoms of listerial meningitis occur about four days after the flu-like symptoms and include fever, personality change, uncoordinated muscle movement, tremors, muscle contractions, seizures, and slipping in and out of consciousness.

Late-onset manifestations

The newborn child may present with late-onset syndrome causes the development of Meningitis between birth and third week of life. It is often caused by serotype IV band has a significant mortality rate.

Listeriosis and Pregnancy

Pregnant women – They are about 20 times more likely than other healthy adults to get Listeriosis. About one-third of listeriosis cases happen during pregnancy.

Laboratory Diagnosis

  • Specimens- Blood, CSF, amniotic fluid, pus, swabs from cervical and vaginal secretions, meconium, cord blood, etc.
  • Direct Microscopy-In Gram stain smears of CSF sediments, the bacteria may be seen as Gram-positive coccobacilli.

  • Culture- Blood agar shows small colonies surrounded by a narrow zone of β-hemolysis as shown above picture.
  • Biochemical tests-Catalase test: Positive, Indole, oxidase, and urea hydrolyzation test: Negative. It ferments glucose and maltose with acid production and CAMP test: Positive.
  • The motility test of  L. monocytogenes shows tumbling motility.

Treatment of Listeriosis

Listeriosis is a serious disease requiring hospitalization. A combination of antibiotics is given intravenously through a small straw-like catheter. When infection occurs during pregnancy, antibiotics must be given promptly to the mother to prevent infection of the fetus or newborn. The duration of antibiotic treatment is at least two weeks. Even with prompt treatment, some infections result in death. antibacterial drugs in use are ampicillin, erythromycin, intravenous trimethoprim-sulfamethoxazole (Co-trimoxazole), cephalosporins, and fluoroquinolones are not active against l.monocytogens. A combination of gentamycin and ampicillin on a clinical basis.

Prophylaxis for Listeriosis

Completely cook all meats and eggs. Carefully wash raw vegetables before eating. Keep raw meat away from raw vegetables and prepared foods. After cutting raw meat, wash the cutting board with detergent before using it for vegetables. Avoid drinking unpasteurized milk or foods made from such milk. Wash hands thoroughly after handling raw meat. Follow the instructions on food labels. Observe food expiration dates and storage conditions.


  1. Experimental inoculation of  L. monocytogenes in rabbits causes a marked monocytosis ( high number of monocytes than normal range in the blood) and thus the name monocytogenes.
  2. All the above eight species may cause disease in animals. Only L. monocytogenes and very rarely L. ivanovii to be associated with human disease.
  3. The characteristics motility and cultural characteristics of L. monocytogenes are usually sufficient to identify it without the need to use many biochemical tests.
  4.  Bacteria Erysipelothrix rhusiopathiae, L. monocytogenes, and Corynebacterium species can be differentiated on the basis of motility test, catalase test, and production of hydrogen sulfide. Among them,  Erysipelothrix rhusiopathiae only produces hydrogen sulfide (H2S) while L. monocytogenes are only motile. Corynebacterium species is negative for those tests.

Further Readings

  1. Bailey & Scott’s Diagnostic Microbiology. Editors: Bettey A. Forbes, Daniel F. Sahm & Alice S. Weissfeld, 12th ed 2007, Publisher Elsevier.
  2. Clinical Microbiology Procedure Handbook Vol. I & II, Chief in editor H.D. Isenberg, Albert Einstein College of Medicine, New York, Publisher ASM (American Society for Microbiology), Washington DC.
  3. Colour Atlas and Textbook of Diagnostic Microbiology. Editors: Koneman E.W., Allen D.D., Dowell V.R. Jr, and Sommers H.M.
  4. Jawetz, Melnick and Adelberg’s Medical Microbiology. Editors: Geo. F. Brook, Janet S. Butel & Stephen A. Morse, 21st ed 1998, Publisher Appleton & Lance, Co Stamford Connecticut.
  5. Mackie and Mc Cartney Practical Medical Microbiology. Editors: J.G. Colle, A.G. Fraser, B.P. Marmion, A. Simmous, 4th ed, Publisher Churchill Living Stone, New York, Melborne, Sans Franscisco 1996.
  6.  Manual of Clinical Microbiology. Editors: P.R. Murray, E. J. Baron, M. A. Pfaller, F. C. Tenover and R. H. Yolken, 7th ed 2005, Publisher ASM, USA
  7.  Textbook of Diagnostic Microbiology. Editors: Connie R. Mahon, Donald G. Lehman & George Manuselis, 3rd edition2007, Publisher Elsevier.
  8. Topley & Wilsons’ Principle of Bacteriology, Virology, and immunology. Editors: M.T. Parker & L.H. Collier, 8th ed 1990, Publisher Edward Arnold publication, London.
  9. Medical Microbiology-The Practice of Medical Microbiology Vol-2-12th Edn. –Robert Cruickshank
  10. District Laboratory Practice in  Tropical Countries  –  Part-2-   Monica Cheesebrough-   2nd Edn Update
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