Possible pseudoresistance of Streptococcus pneumoniae to penicillin G in a patient with a mixed pneumococcus-Staphylococcus aureus pneumonia.

We report the case of a 53-year-old woman with a mixed pneumococcus-staphylococcus pneumonia, in which both organisms were recovered from both sputum and blood. Streptococcus pneumoniae persisted in sputum 48 hours after initiation of high-dose intravenous penicillin G. When nafcillin was substituted for penicillin G, both pneumococci and staphylococci were eradicated from blood and sputum. This strain of Streptococcus pneumoniae was highly susceptible to penicillin G, but the associated strain of Staphylococcus aureus was not. The staphylococcus produced large amounts of a penicillin -degrading betalactamase . We reviewed the records of ten cases of pneumococcus pneumonia from the Wayne State University-Detroit Medical Center admitted from March 1978 to April 1981, in which sputum cultures were repeated within one to ten days after penicillin G had been initiated. At second cultures of sputum, Streptococcus pneumoniae was recovered in none of these latter cases. We further showed that on a blood agar culture plate in the presence of penicillin G, a beta-lactamase positive strain of Staphylococcus aureus allowed growth of Streptococcus pneumoniae. Therefore, despite penicillin therapy, Staphylococcus aureus in sputum may facilitate the persistence of Streptococcus pneumoniae.

Studies of in vitro synergy between several beta-lactam and aminoglycoside antibiotics against endocarditis strains of Pseudomonas aeruginosa.

Ten strains of Pseudomonas aeruginosa isolated from patients with endocarditis (1969-1975) and eight similar strains (1980) were assayed for minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) to several aminoglycosides (gentamicin, tobramycin, amikacin) and beta-lactam antibiotics (ticarcillin, piperacillin, azlocillin, moxalactam and MKO 787). In vitro synergy (1969-1975 series) between beta-lactam and aminoglycoside antibiotics was shown uniformly with azlocillin (100 per cent) followed by moxalactam (80 per cent), piperacillin and ticarcillin (66 per cent) and MKO 787 (13.3 per cent). Results were similar in 1980. Synergy of azlocillin was demonstrated with five strains previously not showing synergy between carbenicillin and an aminoglycoside. In 1980 four of eight patients infected with pseudomonads that were not synergistically affected in vitro were refractory to treatment with the piperacillin-aminoglycoside combination. In vitro synergy of the infecting strain is necessary for successful medical treatment of patients with P. aeruginosa infective endocarditis.

Current problems in the treatment of infective endocarditis due to Pseudomonas aeruginosa.

Mortality from pseudomonas infective endocarditis remains high despite optimal use of available antibacterial agents. Infection of the tricuspid valve is subacute, but involvement of the mitral or aortic valve precipitates more serious disease. Most valvular infections are due to a single pseudomonad immunotype, but 20% of cases are mixed infections. Antimicrobial susceptibility tests and tests of synergy by beta-lactam and aminoglycoside antibiotics in combination were performed on 30 isolates of Pseudomonas aeruginosa. Azlocillin was the most effective beta-lactam in combination with an aminoglycoside; MKO 787 was least effective. Among the aminoglycosides tested, netilmicin was the most effective. Medical treatment combined with valvulectomy (without valve replacement) is now standard treatment for refractory right-sided endocarditis at this medical center. A high dose of aminoglycoside in combination with a beta-lactam has proved efficacious. For left-sided infection, immediate valve replacement accompanied by a six-week course of the high dose-combined drug regimen is recommended. Newer beta-lactam antibiotics, such as piperacillin, may be limited in usefulness due to beta-lactamase inactivation.