Antimicrobial Susccptibilit\ of the Legionnaires' Disease Bacterium 



All clinical and environmental strains of the LDB tested produce a /3-lactamase which is 

 more active on cephalosporins than on penicillins (13). The greater activity of cefoxitin on these 

 organisms probably reflects its resistance to the j3-lactamase produced by the LDB. 



On the other hand, cefamandole, a cephalosporin that is resistant to some (3-lactamases of 

 gram-negative bacilli, is not resistant to the LDB j3-lactamase. As new |3-lactum antibiotics are 

 synthesized, they should be tested for susceptibility to the enzyme of the LDB. If they are 

 resistant, they should be candidates for clinical trials of treatment for Legionnaires' disease. 



The in vitro results show that minocycline and doxycycline are more active than tetra- 

 cycline. All the drugs (except for cotrimoxazole) that were active on the LDB on the basis of 

 MICs were bactericidal at levels comparable to the MIC. The minimum bactericidal concentra- 

 tions (MBCs) for cotrimoxazole were 33-fold greater than the MIC, but were still within achiev- 

 able blood levels. 



In vitro susceptibility results must be interpreted cautiously because of the difficulty in 

 growing the organisms. None of the artificial media supports the rapid growth that is needed for 

 susceptibility tests. 



One reason for the lack of correlation between the in vitro and in vivo results may be the 

 relative differences in the intracellular bactericidal action of the drugs. The LDB are frequently 

 found within macrophages in lung specimens from patients who died with Legionnaires' disease 

 (/) and within peritoneal macrophages in experimentally infected guinea pigs (Chandler. F.. 

 cited in 6). This may explain why rifampin and erythromycin are effective both in vitro and in 

 vivo, whereas an aminoglycoside is effective only in vitro. 



All strains of LDB that have been tested have shown similar susceptibility patterns. Because 

 of this, and because the methods for performing susceptibility tests are far from optimal, we 

 recommend that susceptibility tests not be performed routinely. Reference laboratories, such as 

 ours, can monitor future isolates for changes in susceptibility. 



All the LDB have produced a jS-lactamase when tested by a chromogenic cephalosporin test 

 (see Appendix) but not by the starch-iodine or acidometric techniques {13). The chromogenic 

 cephalosporin test is very simple to perform. Although jS-lactamase data may not be useful in 

 formulating therapeutic strategy, tliey may be useful in characterizing the LDB. 



REFERENCES 



1. Blackmon. John A.. M.D. Hicklin, F.W. Chandler, and the Special Expert Pathology Panel. 1978. Legion- 

 naires" disease, pathological and historical aspects of a 'new' disease. Arch. Patiiol. Lab. Med. 102:337-343. 



2. Brenner, D.J., A.G. Steigerwalt, R.E. Weaver, J.E. McDade, J.C. Feeley. and M. Mandel. 1978. Classifi- 

 cation of the Legionnaires' disease bacterium: An mterim report. Current Microbiology 1:71-75. 



3. Center for Disease Control. 1977. Follow-up on Legionnaires' disease-Pennsylvania. Morbidity and Mortal- 

 ity Weekly Report 26:152. 



4. Center for Disease Control. 1978. Legionnaires" disease - United States. Morbidity and Mortality Weekly 

 Report 27:439-441. 



5. Eraser, D.W., I.E. Tsai, W. Orenstem. W.E. Parkin, H.J. Beecham, R.G. Sharrar, J. Harris. G.E. Mallison, 

 S.M. Martin, J.E. McDade, C.C. Shepard. P.S. Brachman, and the field investigation team. 1977. Legion- 

 naires" disease: Description of an epidemic of pneumonia. N. Engl. J. Med. 297:1 189-1197. 



6. Eraser, D.W., l.K. Wachsmuth, C. Bopp. J.C. Feeley. and I.E. Tsai. 1978. Antibiotic treatment of guinea 

 pigs infected with agent of Legionnaires" disease. Lancet i: 175-178. 



7. Keys, T.F. 1977. A sporadic case of pneumonia due to Legionnaires' disease. Mayo Clin. Proc. 52:657-660. 



8. Lewis, V.J., W.L. Thacker, C.C. Shepard. and J.E. McDade. 1978. In vivo susceptibility of the Legionnaires' 

 disease bacterium to ten antimicrobial agents. Antimicrob. Agents Chemother. 13:419-422. 



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