In a 1-month-old infant with suspected meningitis, which preliminary tests would help differentiate L. monocytogenes from Streptococcus spp.?

To differentiate Listeria monocytogenes from Streptococcus spp. in a 1-month-old infant suspected of having meningitis, the use of hanging-drop motility and catalase tests is significant.

Listeria monocytogenes is unique among the bacterial pathogens considered in this context due to its motility, which can be observed using a hanging-drop technique. This test allows for the visualization of the typical tumbling motility characteristic of Listeria at an optimal growth temperature of 25°C. In contrast, Streptococcus spp. are generally non-motile and do not exhibit such behavior.

The catalase test further aids in distinguishing between these two genera. Listeria monocytogenes is catalase-positive, which means it produces the enzyme catalase, leading to the breakdown of hydrogen peroxide into water and oxygen. This reaction can be easily observed as bubbling when hydrogen peroxide is added to a culture. In contrast, Streptococcus spp. are catalase-negative and do not produce this enzyme, resulting in no observable reaction.

Together, these two tests provide reliable preliminary information to differentiate Listeria monocytogenes from various species of Streptococcus in a clinical setting where meningitis is suspected in an infant.