Abstract
Listeria monocytogenes is a foodborne pathogen that typically presents β-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.
Highlights
Academic Editor: ElenaListeria monocytogenes is a ubiquitous bacterium that is sporadically found in various food products, both processed and fresh or raw
In another study, where 181 L. monocytogenes samples originating from human clinical, animal clinical, food, and environmental sources were included, the authors found four isolates that showed no hemolysis on blood agar plates
The largest study performed to date determined that prevalence rate of nonhemolytic L. monocytogenes reaches approximately 0.1% [26]
Summary
Listeria monocytogenes is a ubiquitous bacterium that is sporadically found in various food products, both processed and fresh or raw. L. monocytogenes and, in some of them, the lack of β-hemolysis presented by collected isolates would probably be considered an exclusive criterion, as authors confirmed species identification (apart from testing other traits) with β-hemolytic tests [17,18,19,20]. It was reported that strains responsible for clinical cases do not always present high hemolytic activity [31], which indicates the importance of monitoring (e.g., in food industry) both hemolytic and nonhemolytic strains of L. monocytogenes. In another study, where 181 L. monocytogenes samples originating from human clinical, animal clinical, food, and environmental sources were included, the authors found four isolates that showed no hemolysis on blood agar plates. L. monocytogenes isolates collected from milk products during routine sampling presented a nonhemolytic phenotype. Most of the nonhemolytic L. monocytogenes strains belong to the same serovar 1/2a, it is worth mentioning that this is one of the serovars (along with 1/2b, 1/2c, and 4b) most frequently isolated in general [42,43,44]
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