Nonhemolytic Listeria monocytogenes-Prevalence Rate, Reasons Underlying Atypical Phenotype, and Methods for Accurate Hemolysis Assessment

Listeria monocytogenes is a foodborne pathogen that typically presents beta-hemolytic activity. However, there are literature reports indicating that L. monocytogenes strains are sometimes nonhemolytic or their zones of hemolysis are perceivable only after removal of the colonies from the agar plate. Nonhemolytic L. monocytogenes are most commonly encountered in food products, but some have also been detected in clinical samples. Usually, atypical bacteria of this species belong to serotype 1/2a. Mutations of the prfA gene sequence are the most common reason for changed phenotype, and mutations of the hly gene are the second most common cause. There are also reports that the methodology used for detecting hemolysis may influence the results. Sheep or horse blood, although most commonly used in modern studies, may not allow for the production of clear hemolytic zones on blood agar, whereas other types of blood (guinea pig, rabbit, piglet, and human) are more suitable according to some studies. Furthermore, the standard blood agar plate technique is less sensitive than its modifications such as bilayer or top-layer (overlay) techniques. The microplate technique (employing erythrocyte suspensions) is probably the most informative when assessing listerial hemolysis and is the least susceptible to subjective interpretation.

Automated summary

Listeria monocytogenes is usually beta-hemolytic, but there are cases where it is nonhemolytic or shows hemolysis only after removal from agar plates. Nonhemolytic strains are commonly found in food products and sometimes in clinical samples. Mutations in the prfA gene sequence are the most common cause of changed phenotype, followed by mutations in the hly gene. The method used for detecting hemolysis and the type of blood used may influence the results.