An intracellular gram positive rod – not many of those, apart from in probiotic drinks!  Resistant to cold and salt, so particularly a problem in ready to eat foods eg deli meats, hot dogs (unless steaming hot), cheese esp soft (incl blue veined, but excl mozzarella), raw and cooked poultry, ice cream, raw vegetables, raw and smoked fish (unless in shelf stable form). Melons and hummus have been sources in US.  In adults, tends to affect those with underlying health problems.

Infection in pregnancy often undiagnosed. May cause preterm labour or intrauterine death. Infection at birth may be severe, with classic fine papular rash, widespread microabscesses and granulomas, and bacteria visible on gram stain of the meconium. Or infection may be late in onset eg 1-2 weeks, with meningitis (note low counts cf other causes), else endocarditis, osteomyelitis etc.

Preterm meconium staining of the amniotic fluid (MSAF) is a “classic” feature – was observed in 4.3% of infants below 33/40. No maternal or infant listeriosis was identified in any of the 1000 cases. MSAF was associated with prolonged rupture of the membranes and severe (grade 3/4) intraventricular haemorrhage (OR 2), not sepsis or mortality. (Simpsons, Arch Dis Child Fet 2004)

Surveillance study of bacterial meningitis in infants aged <90 days in the UK 2010-11 showed that then usual three bacteria (GBS, E. Coli and Listeria) remained dominant, their frequency varied significantly by month of life. In the first 30 days of life. L. monocytogenes was the third most common bacteria, responsible for 6% of cases. The median age of meningitis due to L. monocytogenes was 13 days (IQR 3–18 days) with the oldest infant being 29 days; Listeria meningitis was therefore not seen beyond the 1st month of life. Of the 11 cases of Listeria meningitis, a good number (although a minority) were preterm and most first became unwell when at home. 2 cases had serious complications but no deaths.

Public Health England have published 24 years data on listeria septicaemia and meningitis. 97% of all cases presented in the first 30 days of life. Bacteraemia is more common but tends to be early onset (<7 days of age) whereas most meningitis were late onset.

It is also prudent to consider the possibility of Listeria infection in older infants (and therefore add amoxicillin) if:

• Gram-positive rods are seen in the cerebrospinal fluid,
• if the infant is immunocompromised
• or if the clinical response to empirical therapy is suboptimal

Treat with high dose amoxicillin/ampicillin. Gentamicin is synergistic but does not penetrate intracellular compartment (or CSF) – can be stopped after a week assuming good clinical improvement. For allergic, TMP-SMX (Septrin) is best alternative! Cephalosporins are useless! Treat for 2 weeks if no meningitis, at least 3 weeks if meningitis, longer if abscesses or heart involvement.

[Okike, Arch Dis Child 2015;100:426-431]