Listeria monocytogenes - Culture; Molecular diagnosis (PCR); Molecular serotyping (1/2a, 1/2b, 1/2c, 3a, 3b, 3c and 4b)


Information 30-03-2014.


Listeria monocytogenes infection is a zoonosis. Direct contact with animals is not the main route of human infection, but most of the time the infection is acquired through an indirect mechanism with some contaminated food. Foods most involved are those of animal origin, such as unpasteurized dairy products, which have been contaminated by animals, as with fresh cheeses.


Infections with most impact are those that occur in pregnant women, which usually causes an asymptomatic process, gastroenteritis, or a slight fever, similar to flu, with accompanying signs of malaise , and myalgias, sometimes with higher fever and chills. In this phase the microorganism can reach the placenta or amniotic fluid affected fetus. Listeria monocytogenes is an intracellular bacterium, which usually infect cells from one to another, so that the antibody response, if there is minimal. Through infected cells reach the placenta and infect the fetus from this or amniotic fluid.


Neonatal listeriosis is acquired during pregnancy transplacental during delivery, or after it. Intrauterine infection leads to the spread of the organism in the fetus, with granuloma formation in the liver, spleen, adrenals, lymph tissue, lung, brain and skin. When skin involvement appears and the fetus exhibits a rash, the process is called infantiseptica granulomatosis.


In the mother, if infection occurs early in pregnancy during the first or second trimester, and passes to the fetus it causes abortions or fetal death with maceration quite a fetus to be removed.


If infection occurs later, it induces premature birth (preterm), stained amniotic fluid meconium, and born infected with signs of neonatal sepsis or meningitis, cyanosis, apnea, respiratory distress and pneumonia, and a pattern reticulonodular in chest x-ray. When infection occurs in the neonatal period, the signs and symptoms manifest soon, within the first 7 days of life, and preferably the first or second day.


Fetal listeriosis, is mainly due to aspiration or swallowing infected amniotic fluid during pregnancy. In postnatal listeriosis, acquired during birth, the cause lies in the aspiration of vaginal secretions during birth, manifesting itself in the early days of birth with respiratory distress, shock , and a fulminant course. If the infection occurs in late gestation the fetus can I be born infected without showing symptoms or signs septic born.


The high mortality rates (35-55%) occur in premature infants, the existence of micro - abscesses or granulomas containing the microorganism widely distributed at necropsy lung, liver and spleen (granulomatosis infantiseptica).


When the infection is acquired during birth from the mother with an infected birth canal (cervix or vagina), infection manifested with septicemia and / or meningitis several days or even weeks after birth.


The recommended treatment is associated with gentamicin ampicillin for 14 days, or 21 days in case there meningitis. It should be remembered that Listeria monocytogenes is resistant to third - generation cephalosporins, so if you want to use cephalosporins to cover other etiology, should be associated with ampicillin. When there is ampicillin resistance, which are rare, the association of gentamicin with vancomycin should be used.




Listeria monocytogenes is a short gram - positive bacillus, not sporulated, mobile, facultative anaerobic, whose isolation is favored at 4 ° C. 4 antigenic types (1 to 4) are known, with serotypes 1 and 4 the most frequent.


For diagnosis the following alternatives may be considered, with the comments that we present for each of them:


Culture isolation:


Culture methods, with isolation and identification, are slow, so try replaced by molecular methods. The only advantage of growing bacteria is available for testing antimicrobial susceptibility.


In pregnant women with symptoms of fever, without further explanation:


  • Blood culture, cervix secretion, amniotic fluid or if it can get.

In the newborn:


  • Blood culture, cerebrospinal fluid, gastric aspirate, meconium, or tissue sample supposedly infected tissue supposedly.

In the mother:


  • In many cases of neonatal listeriosis, in 30 to 40% of pregnant women Listeria monocytogenes can be detected in cervical or vaginal secretion.


Molecular detection by PCR


  • Sensitive and specific method, recommended for its speed, compared to culture and identification.


Detection antibody:


  • Useless. Not recommended realization that negativity does not exclude the existence of a neonatal listeriosis, or an infection or carrier status of the mother or mother.

Types of culture sample or PCR:


Any of the above depending on when it is done:


  • Pregnant with suspected infection: blood, exudate cervix, vaginal discharge, amniotic fluid obtained by amniocentesis.
  • Gestating without suspected infection to exclude genital colonization exudate cervix, vaginal discharge.
  • Mother newborn with suspected neonatal listeriosis: exudate cervix and vaginal discharge.
  • Newborn with suspected neonatal listeriosis: blood (when there is suspicion of sepsis), cerebrospinal fluid (when there is suspicion of meningitis), puncture or biopsy granulomas if infantiseptica granulomatosis.


Type sample container


            Sterile, suitable for the type of samples collection


Pre-analytical conditions


  • Condition: freezing (> 24 hours for shipping), refrigeration (less than 24 hours before shipping).
  • Shipping: refrigerated container within biological safety, and within isothermal containers with cold accumulator (freezable pack).




  • PCR, or culture, as requested. Cultivation offers the advantage of allowing perform antimicrobial susceptibility.
  • Molecular serotyping to differentiate the seven major serotypes 1 / 2a, 1 / 2b, 1 / 2c, 3a, 3b, 3c and 4b.


Result Format


  • Positive or negative result.


Delivery term


  • Within 48 hours (PCR); 72 hours (culture).


Cost of the test


Consult .