Legionella pneumophila sp. nov.; The Legionnaires' Disease Bacterium 



workers find that members of a given species are at least 70% related (1. S. 76, J7). It lias been 

 our experience that relatedness among strains of a species remains above 60% when reactions are 

 done at the stringent 75°C incubation temperature. Furthemiore, divergence in DNA relatedness 

 between strains of a species is almost always less than 6% (1, J). Strains of LDB were 70% to 99% 

 related to Philadelphia 1 (Table 3). Divergence in related sequences was 3% or less, and re- 

 latedness remained almost unchanged in 75°C reactions. Thus on the basis of these tests all of 

 these LDB isolates belong to a single species. 



Table 3. 

 DNA relatedness among 15 strains of LDB 



a Figures in parentheses are the actual % of DNA reassocialion in homologous Philadelphia 1 reactions, and the Tp, (temperature 

 at which 50% of labeled DNA is detected as single-stranded) for homologous, reassociated DNA. % divergence is the decrease in 

 Tni (in °C) of heterologous DNA duplexes compared to that of the homologous DNA duplexes. It can be expressed as % 

 because each degree decrease in thermal stability is caused by approximately 1% unpaired bases in double-stranded DNA {3). 



The first four strains of^ LDB isolated were called Philadelphia 1, 2, 3, and 4. Strains 

 subsequently isolated were also identified by city to distinguish them from the Philadelphia 

 strains. This convention has now been adopted at CDC, and all strains are designated by city of 

 origin and the number of the isolate from that city. For instance, Flint 1 and Flint 2 were both 

 isolated in Flint, Michigan. If a strain has a city designation without a number, it can be assumed 

 that this is the first isolate from that city: Albuquerque = Albuquerque 1. Specific culture 

 collection numbers are put in parentheses after the city name and number; Flint 1 (D871 1), Flint 

 2 (El 03 5), etc. 



LDB strains Philadelphia 2, 3, and 4 are very closely related to Philadelphia 1. This is not 

 surprising since these strains were all isolated from the same outbreak. The strain from the 

 Burlington outbreak and strains from sporadic cases were also very closely related to Philadelphia 

 1. 



63 



