Therapeutic failure in pneumonia caused by a tolerant strain of Staphylococcus aureus

When antibiotics fail: a clinical and microbiological perspective on antibiotic tolerance and persistence of Staphylococcus aureus

Staphylococcus aureus is a major human pathogen causing a vast array of infections with significant mortality. Its versatile physiology enables it to adapt to various environments. Specific physiological changes are thought to underlie the frequent failure of antimicrobial therapy despite susceptibility in standard microbiological assays. Bacteria capable of surviving high antibiotic concentrations despite having a genetically susceptible background are described as 'antibiotic tolerant'. In this review, we put current knowledge on environmental triggers and molecular mechanisms of increased antibiotic survival of S. aureus into its clinical context. We discuss animal and clinical evidence of its significance and outline strategies to overcome infections with antibiotic-tolerant S. aureus.

Clinical relevance of bacteriostatic versus bactericidal mechanisms of action in the treatment of Gram-positive bacterial infections

The distinction between bactericidal and bacteriostatic agents appears to be clear according to the in vitro definition, but this only applies under strict laboratory conditions and is inconsistent for a particular agent against all bacteria. The distinction is more arbitrary when agents are categorized in clinical situations. The supposed superiority of bactericidal agents over bacteriostatic agents is of little relevance when treating the vast majority of infections with gram-positive bacteria, particularly in patients with uncomplicated infections and noncompromised immune systems. Bacteriostatic agents (e.g., chloramphenicol, clindamycin, and linezolid) have been effectively used for treatment of endocarditis, meningitis, and osteomyelitis--indications that are often considered to require bactericidal activity. Although bacteriostatic/bactericidal data may provide valuable information on the potential action of antibacterial agents in vitro, it is necessary to combine this information with pharmacokinetic and pharmacodynamic data to provide more meaningful prediction of efficacy in vivo. The ultimate guide to treatment of any infection must be clinical outcome.

Etiological diagnosis of bacterial pneumonia by gram stain and quantitative culture of expectorates. Leukocytes or alveolar macrophages as indicators of sample representativity

Sputum samples from 151 patients admitted to Roslagstull Hospital for Infectious Diseases, Stockholm, from Sept. 1978 through May 1979 with acute community-acquired lower respiratory tract disease and roentgenological evidence of acute pneumonia were examined by direct microscopy of gram-stained smears and semiquantitative culture. It was carefully noted if the specimen was collected before or after initiation of antibiotic therapy. For an estimate of the suitability of the samples for bacteriological examination 2 criteria were applied: (i) presence of alveolar macrophages, and (ii) purulence, i.e. a ratio leukocytes/squamous epithelial cells of greater than 5. The latter was found to be a good indicator of sample suitability, while the presence of macrophages was not. Of the 266 samples examined 76% were deemed purulent. Potentially pathogenic bacteria in numbers of greater than or equal to 10(5) colony forming units/ml were found in 67% of the purulent sputum samples obtained before antibiotic therapy but in only 36% if such treatment had already been started. Pneumococci were isolated from 52% of pre-treatment samples but from only 8% after treatment. H. influenzae was found as often in post-treatment samples (17%) as in pre-treatment ones (15%) and enteric gram-negative rods twice as often in post-treatment samples (11 vs. 6%). The use of gram-stained smears was a valuable aid in the interpretation of the culture results and the results could be made available to the clinician within minutes after receipt of the specimen. The results were in agreement with those of the cultures for about 75% of the purulent samples.