Group B streptococci can be distinguished from other beta-hemolytic streptococci by their production of a substance called the
CAMP factor. This term is an acronym for the names of the investigators who first described the factor: Christie, Atkins, and Munch-Petersen. The substance is a peptide that acts together with the beta-hemolysin produced by some strains of Staphylococcus
aureus, enhancing the effect of the latter on a sheep blood agar plate. This effect is sometimes referred to as synergistic hemolysis (see colorplate 30).
||To differentiate group B from group A streptococci by the effect of the group B CAMP factor and
by a serological method
||Demonstration sheep blood agar plate, streaked at separate points with Staphylococcus aureus, group B
streptococci, and group A streptococci
Solution with extracted antigen of beta-hemolytic Streptococcus (prepared by instructor)
Latex test kit for serological typing
Procedures (Steps 1–4 to be followed by instructor)
- With an inoculating loop, streak a strain of S. aureus down the center of a blood agar plate. (ATCC 25923 or other strain
known to produce beta-hemolysin is used; 5% sheep blood agar is needed.)
- On one side of the plate, inoculate a strain of group B Streptococcus by making a streak at a 90° angle, starting 5 mm away
from the S. aureus and extending outward to the edge of the agar (see diagram).
- On the other side of the plate, inoculate a strain of group A Streptococcus, again at a 90° angle from the S. aureus, as in step
2. This streak should not be directly opposite the group B inoculum (see diagram).
- Incubate the plate aerobically at 35°C for 18 to 24 hours.
- The student should confirm the isolate’s identity by the serological test. Using the extracted antigen solution, follow the
procedures in steps 4 and 5 of Experiment 21.1.
- Observe the area of hemolysis surrounding the S. aureus streak. At the point adjacent to the streak of group B streptococci,
you should see an arrowhead-shaped area of increased hemolysis indicating production of the CAMP factor (review colorplate 30). There should be no change in the hemolytic zone adjacent to the streak of group A streptococci, most
strains of which do not produce the CAMP factor.
- Although most group A streptococci give a negative CAMP test, some have been reported to be positive, especially when
the test plate has been incubated anaerobically rather than aerobically. The bacitracin disk test may be useful in
distinguishing the latter from group B streptococci. Conversely, however, occasional strains of group B streptococci may be
bacitracin susceptible. In such cases, a serological grouping method may be required for final identification. The following
scheme may be followed by diagnostic laboratories reporting the results of these tests.
- With which latex reagent did you obtain a positive result? Group