CLINICAL PHARMACOLOGY
Penicillin V exerts a bactericidal action against
penicillin-sensitive microorganisms during the stage of active multiplication.
It acts through the inhibition of biosynthesis of cell-wall mucopeptide. It is
not active against the penicillinase-producing bacteria, which include many
strains of staphylococci. The drug exerts high in vitro activity against
staphylococci (except penicillinase-producing strains), streptococci (groups A,
C, G, H, L and M), and pneumococci. Other organisms sensitive in vitro to penicillin
V are Corynebacteriumdiphtheriae, Bacillus anthracis, Clostridia,
Actinomycesbovis, Streptobacillusmoniliformis, Listeria monocytogenes,
Leptospira, and Neisseria gonorrhoeae. Treponemapallidum is
extremely sensitive.
The potassium salt of penicillin V has the distinct
advantage over penicillin G in resistance to inactivation by gastric acid. It
may be given with meals; however, blood levels are slightly higher when the
drug is given on an empty stomach. Average blood levels are two to five times
higher than the levels following the same dose of oral penicillin G and also
show much less individual variation. Once absorbed, penicillin V is about 80%
bound to serum protein. Tissue levels are highest in the kidneys, with lesser
amounts in the liver, skin, and intestines. Small amounts are found in all
other body tissues and the cerebrospinal fluid. The drug is excreted as rapidly
as it is absorbed in individuals with normal kidney function; however, recovery
of the drug from the urine indicates that only about 25% of the dose given is
absorbed. In neonates, young infants, and individuals with impaired kidney
function, excretion is considerably delayed.
Microbiology
Susceptibility Testing
Diffusion Techniques
Quantitative methods that require measurement of zone
diameters provide reproducible estimates of the susceptibility of bacteria to
antimicrobial compounds. One such standardized procedure2,4 which
has been recommended for use with disks to test susceptibility of organisms to
penicillin uses the 10 Unit (U) penicillin disk. Interpretation involves the
correlation of the diameters obtained in the disk test with the minimum
inhibitory concentration (MIC) for penicillin.
Reports from the laboratory providing results of the
standard single-disk susceptibility test with a 10 U penicillin disk should be
interpreted according to the criteria provided in Table 1.
Dilution Techniques
Quantitative methods that are used to determine minimum
inhibitory concentrations (MICs) provide reproducible estimates of the
susceptibility of bacteria to antimicrobial compounds. One such standardized
procedure3,4 uses a standardized dilution method (broth or agar) or
equivalent with penicillin powder. The MIC values obtained should be
interpreted according to the criteria provided in Table 1.
Table 1: SUSCEPTIBILITY TEST INTERPRETIVE CRITERIA
Pathogen |
Susceptibility Test Result Interpretive Criteria |
Disk Diffusion (Zone diameter in mm) |
Mil C (M |
limal Inhibitory loncentration IC in mcg/mL) |
S |
I |
R |
S |
I |
R |
Staphylococcus spp. |
≥ 29 |
- |
≤ 28 |
≤ 0.12 |
- |
≥ 0.25 |
Streptococcus spp. (beta-hemolytic group) |
≥ 24 |
- |
- |
≤ 0.12 |
- |
- |
Streptococcus pneumoniae (non-meningitis isolates) |
|
|
|
≤ 0.06 |
0.12-1 |
≥ 2 |
A report of “susceptible” indicates that the pathogen is
likely to be inhibited by usually achievable concentrations of the
antimicrobial compound in the blood. A report of “Intermediate” (I) indicates
that the result should be considered equivocal, if the microorganism is not
fully susceptible to alternative, clinically feasible drugs, the test should be
repeated. This category implies possible clinical applicability in body sites
where the drug is physiologically concentrated or in situations where high dosage
of the drug can be used. This category also provides a buffer zone that
prevents small uncontrolled technical factors from causing major discrepancies
in interpretation. A report of “resistant” indicates that the pathogen is not
likely to be inhibited if the antimicrobial compound in the blood reaches the
concentrations usually achievable; other therapy should be selected.
Quality Control
Standardized susceptibility test procedures require the
use of laboratory control microorganisms2,3,4. The 10 U penicillin disk
and the standard penicillin powder should provide respectively the following zone
diameters and MIC values in these laboratory test quality control strains:
Table 2: ACCEPTABLE QUALITY CONTROL RANGES
Microorganism |
Acceptable Quality Control Ranges |
Disk Diffusion (Zone diameter ranges in mm) |
Minimal Inhibitory Concentration Range (MIC in mcg/mL) |
Staphylococcus aureus ATCC® 25923 |
26-37 |
|
Staphylococcus aureus ATCC® 29213 |
|
0.25-2 |
Streptococcus pneumoniae ATCC® 49619 |
24-30 |
0.25-1 |
REFERENCES
2. Clinical and Laboratory Standards Institute. Performance
Standards for Antimicrobial Disk Susceptibility Test; Approved
Standard-Eleventh Edition. CLSI document M02-A11. Clinical and Laboratory
Standards Institute, 950 West Valley Road, Suite 2500, Wayne, Pennsylvania
19087, USA, 2012.
3. Clinical and Laboratory Standards Institute. Methods
for Dilution Antimicrobial Susceptibility Test for Bacteria That Grow
Aerobically; Approved Standard-Ninth Edition. CLSI document M07-A9. Clinical
and Laboratory Standards Institute, 950 West Valley Road, Suite 2500, Wayne, Pennsylvania
19087, USA, 2012.
4. Clinical and Laboratory Standards Institute. Performance
Standards for Antimicrobial Susceptibility Testing; Twenty-Second Informational
Supplement. CLSI document M100-S22. Clinical and Laboratory Standards
Institute, 950 West Valley Road, Suite 2500, Wayne, Pennsylvania 19087, USA,
2012.