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The interpretation of streptococcus pneumoniae antimicrobial susceptibilities.

Streptococcus pneumoniae is a common cause of respiratory tract infection. It is the most common bacterial cause of community-acquired pneumonia, sinusitis and otitis media. The emergence of pneumococcal resistance to penicillin and to other commonly prescribed antimicrobials is of specific concern globally and in South Africa. During the last 6 months of 2007, upper respiratory tract isolates' susceptibility to penicillin in South Africa varied between 27% and 60% geographically, while the susceptibility of lower respiratory tract isolates varied between 54% and 93%. (1)

Pneumococcal resistance to macrolides is another problem, and invasive S. pneumoniae resistant to levofloxacin has recently been described as emerging in South Africa in children receiving quinolones as part of multidrug-resistant tuberculosis therapy. (2)

To complicate matters, in 2008 the Clinical Laboratory Standards Institute (CLSI) revised the guidelines for pneumococcal reporting according to site of infection, as summarised in Table I. (3)


Penicillin and parenteral third-and fourth-generation cephalosporins are reported according to whether the site of infection is meningeal or non-meningeal.

Most laboratories will report susceptibilities of S. pneumoniae according to the minimum inhibitory concentration (MIC) to both penicillin and ceftriaxone. The values obtained will then be interpreted according to whether the site of infection is meningeal, or another site, including inner ear (otitis media), sinus (sinusitis), or lung (pneumonia). In severe pneumococcal otitis media or sinusitis it may be prudent to consult a microbiologist, as current breakpoints may not reflect optimal pharmacodynamics or kinetics for these infections. It is also important to provide adequate clinical information on the pathology request form to enable interpretation of pneumococcal susceptibilities. In practice a distinction will be made between oral and parenteral therapy with the following comments as examples:

Parenteral treatment

All pneumococcal infections (excluding meningitis) with a penicillin MIC of [less than or equal to]2 [micro]g/ml can be treated with IV penicillin 12 million units per day. These isolates can also be considered susceptible to amoxicillin and ceftriaxone. Strains with an intermediate penicillin MIC of 4 [micro]g/ml may require 18-24 million units per day.

Oral treatment

All pneumococcal isolates (excluding meningitis) with a penicillin MIC of [less than or equal to]0.06 [micro]g/ml can be considered susceptible to amoxicillin, amoxicillin-clavulanic acid, cefaclor, loracarbef, cefprozil, cefuroxime and cefpodoxime for approved indications. Isolates with an MIC of 0.12-1 [micro]g/ml should be treated with an increased dosage of amoxicillin or amoxicillin-clavulanic acid, or a different class of antibiotic to which the isolate is susceptible.


(1.) Senekal M. Optimizing antimicrobial therapy for the treatment of respiratory tract infection. Modern Medicine 2008; 33(5): 42-46.

(2.) Von Gottberg A, Klugman KP, Cohen C, et al. Emergence of levofloxacin non-susceptible Streptococcus pneumoniae and treatment for multi-drug resistant tuberculosis in children in South Africa: a cohort observational surveillance study. Lancet 2008; 371: 11081113.

(3.) Clinical Laboratory Standards Institute 2008 M100-S18; 28(1): 126-128.


Clinical Microbiologist, Pathcare Reference Laboratory, Goodwood
Table I. Clinical Laboratory Standards Institute 2008,
breakpoints for reporting S. pneumoniae

 Susceptible Intermediate

IV non-meningitis [less than or equal to] 2 [micro]g/ml 4
IV meningitis [less than or equal to] 0.06 --

Oral pen V [less than or equal to] 0.06 0.12-1
Amoxicillin [less than or equal to] 2 [micro]g/ml 4

Non-meningitis [less than or equal to] 1 2
Meningitis [less than or equal to] 0.5 1

Non-meningitis [less than or equal to] 1 2
Meningitis [less than or equal to] 0.5 1

 Resistant Treatment advice

IV non-meningitis [greater than or equal to] 8
IV meningitis [greater than or equal to] 0.12 Maximum doses
 should be given
Oral pen V [greater than or equal to] 2
Amoxicillin [greater than or equal to] 8

Non-meningitis [greater than or equal to] 4
Meningitis [greater than or equal to] 2 Maximum doses
 should be given
Non-meningitis [greater than or equal to] 4
Meningitis [greater than or equal to] 2
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Article Details
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Title Annotation:More about ... Microbiology
Author:Senekal, Marthinus
Publication:CME: Your SA Journal of CPD
Article Type:Report
Geographic Code:6SOUT
Date:Nov 1, 2008
Previous Article:C-reactive protein and procalcitonin as markers of infection.
Next Article:Laboratory diagnosis of paediatric lower respiratory tract infection.

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