An 8 year old girl with cystic fibrosis was admitted to the hospital with
respiratory distress and increasing sputum production. The physician
obtained an expectorated sputum specimen for culture and susceptibility
testing prior to initiating antimicrobial therapy with high doses of
intravenous tobramycin and ceftazidime. Preliminary examination of the
culture revealed two types of oxidase-positive gram-negative rods, both
with colony morphologies consistent with Pseudomonas aeruginosa. One of
the colony types was mucoid and the other was flat with a dry appearance.
A moderate amount of normal respiratory flora was also isolated.
The patient's symptoms began to lessen after 2 days of therapy and
she was released from the hospital following cessation of antimicrobial
therapy on day 14.
Question:
Should additional biochemical tests be performed to confirm the
identification of the "presumed" colonies as P. aeruginosa?
Answer:
Yes, for both colony types. NCCLS publication, M35-P "Abbreviated
identification of bacteria and yeast; proposed guideline", published in
2000, delineates the minimum set of test results required for
identification of several commonly encountered species of bacteria and
yeast. For P. aeruginosa, these include oxidase-positive colonies of a
gram-negative rod displaying typical smell (described as similar to
Concord grapes or corn tortillas) that exhibit recognizable colony
morphology (metallic or pearlescent, rough, pigmented, or extremely mucoid).
However, isolates from cystic fibrosis (CF) patients should be evaluated
carefully because P. aeruginosa colonies with atypical morphologies (known
as morphotypes) are common among these patients, and some Burkholderia
cepacia isolates from CF patients can resemble P. aeruginosa.
Question:
Is it necessary to perform separate antimicrobial susceptibility tests
on each of the two colony types, or can these colonies be combined into a
single inoculum?
Answer:
We recommend that each colony type be tested separately. While there
has been debate about this among investigators for several years, there
are currently no guidelines that advocate the testing of multiple colony
types together in a single inoculum. In 1994, Morlin et al. reported that
accurate broth microdilution MIC results could be obtained by combining
multiple colony types of P. aeruginosa when sputa contained only
antimicrobial-susceptible isolates. However, when both resistant and
susceptible isolates were present, testing of multiple colony types
together detected resistance only 57% of the time. Although it is
conceivable that the mix of colony types may represent the disease process
present in the lung, it is not clear which morphotype may predominate or
how the organisms may interact. On the other hand, Van Horn reported that
broth microdilution MIC results using an inoculum containing several
colony morphologies correlated 96% of the time with the most resistant
result determined by testing isolates individually. Given that there are
conflicting data, the conservative approach is to test each isolate
separately.
Question:
Should an MIC or disk diffusion method be used for testing P.
aeruginosa from CF patients?
Answer:
Either method is acceptable. In NCCLS M00-S11, "Performance standards
for antimicrobial susceptibility testing: eleventh informational
supplement" (published in January 2001), both MIC and disk diffusion
methods are reported to provide accurate results for testing P. aeruginosa
from CF patients. Specifically, the comments are:
For MIC testing:
"The susceptibility of Pseudomonas aeruginosa isolated from patients
with cystic fibrosis can be reliably determined by the reference agar
dilution or frozen reference broth microdilution methods, but may require
extended incubation up to 24 hours."
For disk diffusion testing:
"The susceptibility of Pseudomonas aeruginosa isolated from patients
with cystic fibrosis can be reliably determined by the disk method, but
may require extended incubation up to 24 hours."
Question:
Is it acceptable to use commercial MIC systems for testing P.
aeruginosa from CF patients?
Answer:
The ability of a commercial susceptibility testing system to provide
accurate MIC results for P. aeruginosa varies from manufacturer to
manufacturer. Therefore, it is important to check the package insert of
the product before testing mucoid strains of P. aeruginosa. The
manufacturers of the systems should provide data to support the use of
their products with P. aeruginosa, particularly mucoid strains. Some
mucoid strains do not grow adequately in automated test systems.
Question:
If either the dry or mucoid isolate is resistant to all agents
suggested by NCCLS for testing P. aeruginosa from CF patients, are there
other antimicrobial agents that could be tested and reported?
Answer:
Yes. The primary anti-pseudomonal agents (in alphabetical order) are:
- aminoglycosides - amikacin, gentamicin, tobramycin
- carbapenems - imipenem, meropenem
- carboxypenicillins - ticarcillin
- extended-spectrum cephalosporins - ceftazidime, cefepime,
cefoperazone
- fluoroquinolones - ciprofloxacin
- monobactam - aztreonam
- ureidopenicillins - piperacillin
If a P. aeruginosa strain is resistant to all of the above agents,
polymyxin B and colistin (also known as polymyxin E) should be considered
for testing. Both agents are active against most strains of P.
aeruginosa;
however, because they diffuse poorly in agar, MIC testing should be
performed. There are no standard criteria for interpretation of either
polymyxin B or colistin; however, in a recent paper by Gales et al., an
MIC of > 4 µg/ml was considered resistant for both agents.
Question:
Should ciprofloxacin be tested and reported on P. aeruginosa even if
the patients are less than 18 years old?
Answer:
Report ciprofloxacin only if your medical staff requests you to do so.
Because P. aeruginosa can be resistant to other anti-pseudomonal agents,
ciprofloxacin is sometimes prescribed in children with CF. This is
addressed in The Red Book 2000 as follows: "after careful assessment of
the risks and benefits for the patient under age 18, use of a
fluoroquinolone can be justified in special circumstances such as 1) no
other oral agent is available, necessitating an alternative drug given
parenterally and 2) infection is caused by multidrug-resistant,
gram-negative, enteric, and other pathogens such as certain Pseudomonas.
The risks and benefits should be explained to the patients and parents."
Question:
How often should the antimicrobial susceptibility profiles of P.
aeruginosa isolates from CF patients be determined?
Answer:
There are no standard recommendations that apply directly to P.
aeruginosa isolates from CF patients. However, it is known that P.
aeruginosa can develop resistance during prolonged antimicrobial therapy.
NCCLS notes in document M100-S11 that resistance can occur within 3 to 4
days of therapy, at which time retesting may be warranted, particularly if
a patient is failing to respond clinically.
Question:
Is there a role for synergy testing on isolates of P. aeruginosa from
CF patients?
Answer:
Although some references suggest that synergy testing may be
appropriate when a P. aeruginosa isolate is resistant to all anti-pseudomonal
agents, such testing remains controversial. Published studies with
clinical data showing that the results of synergy tests improve clinical
outcome of CF patients are difficult to find. Nonetheless, some physicians
find the results of synergy testing of value for patient management.
References:
- American Academy of Pediatrics. Antimicrobial agents and related
therapy. In: Pickering, L.K. ed. 2000 Red Book: Report of the
Committee on Infectious Diseases. 25th ed. Elk Grove Village, IL:
American Academy of Pediatrics. 2000:645-646.
- Burns, J. L., L. Saiman, S. Whittier, D. Larone, J. Krzewinski, Z.
Liu, S. A. Marshall, and R. N. Jones. 2000. Comparison of agar
diffusion methodologies for antimicrobial susceptibility testing of
Pseudomonas aeruginosa isolates from cystic fibrosis patients. J Clin
Microbiol. 38:1818-1822.
- Campbell, P.W. III and L. Saiman. 1999. Use of aerosolized
antibiotics in patients with cystic fibrosis. Chest. 116:775-788.
- Gales, A. C., A. O. Reis, and R. N. Jones. 2001. Contemporary
assessment of antimicrobial susceptibility testing methods for
polymyxin B and colistin: review of available interpretative criteria
and quality control guidelines. J Clin Microbiol. 39:183-190.
- Morlin, G. L., D. L. Hedges, A. L. Smith, and J. L. Burns. 1994.
Accuracy and cost of antibiotic susceptibility testing of mixed
morphotypes of Pseudomonas aeruginosa. J Clin Microbiol. 32:1027-1030.
- NCCLS. 2001. Performance standards for antimicrobial susceptibility
testing: eleventh informational supplement. M100-S11. NCCLS. Wayne,
PA.
- Saiman, L., J. L. Burns, S. Whittier, J. Krzewinski, S. A. Marshall,
and R. N. Jones. 1999. Evaluation of reference dilution test methods
for antimicrobial susceptibility testing of Pseudomonas aeruginosa
strains isolated from patients with cystic fibrosis. J Clin Microbiol.
37:2987-2991.
- Van Horn, K. G. 1993. Mixed-morphotype broth microdilution
susceptibility testing of Pseudomonas aeruginosa from cystic fibrosis
patients. J Clin Microbiol. 31:458-459.
- Wolter, J. M., G. Kotsiou, and J. G. McCormack. 1995. Mixed
morphotype testing of Pseudomonas aeruginosa cultures from cystic
fibrosis patients. J Med Microbiol. 42:220-224.
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