PRINCIPAL INVESTIGATOR.'PROGRAM DIRECTOR: 
Sandra Handwerger, M.D. 
Because of the ubiquitous nature of the D-alanyl-D-alanine 
terminus in bacterial cell walls, the acquisition of vancomycin 
resistance among Gram positive bacteria has been held as highly 
unlikely. (Gram negative bacteria evade the action of vancomycin 
by the presence of an outer membrane which excludes large 
molecules [10]). Reports of vancomycin resistance among Gram 
positive bacteria have been rare. Reviews of published 
susceptibility data from 1956 to 1978 noted that staphylococci 
tested after 1960 were uniformly susceptible, and only rare 
isolates of streptococci were resistant (67) . More recently, 
moderately resistant isolates of Staphylococcus hemolyticus (MIC 
= 8 ug/ml) were recovered from the peritoneal fluid of a patient 
who had received long term vancomycin therapy; upon repeated 
exposure to vancomycin, stable clones with MICs of up to 128 
ug/ml could be selected (59) . Enterococci with vancomycin MICs 
of 512 ug/ml have also recently been isolated from the blood and 
gastrointestinal tract (38,66). In some strains a plasmid (34 to 
38 kilobases) was identified which conferred vancomycin 
resistance when introduced into susceptible enterococci or 
Streptococcus sancjuis (38) . Other strains of vancomycin 
resistant S. faecium have been described which lack plasmids 
(66), suggesting that in some isolates, vancomycin is 
chromosomal ly mediated. 
Although vancomycin resistance has now been described in 
several Gram positive genera, the majority of vancomycin 
resistant isolates causing bacteremia have been identified as 
Leuconostoc species. Most of the affected patients had been 
receiving broad spectrum antibiotics, including vancomycin, or 
had serious underlying diseases (6,30,57). However, the 
isolation of Leuconostoc from the cerebrospinal fluid of a 
previously healthy young woman with meningitis suggests that 
these organisms may be pathogenic for the non-hospitalized , 
non-immunocompromised host as well (7) . 
Most clinical isolates of Leuconostoc are initially 
identified as pneumococci or viridans streptococci, due to their 
colonial morphology and fermentation profile, and are only 
subjected to appropriate testing and identified correctly when 
vancomycin resistance is observed. Production of gas from 
glucose-containing media, which distinguishes leuconostocs from 
other Streptococcaceae , is not routinely performed in clinical 
laboratories (16) . Thus the true frequency of infections due to 
Leuconostoc, and the prevalence of resistance among clinical 
isolates, remains unknown. Clinical isolates identified thus 
far, as well as many strains from agricultural sources, 
demonstrate resistance to vancomycin concentrations as high as 
1000 ug/ml (6,7,8,11,30,49). However, susceptible strains are 
also found among agricultural isolates (MIC N < 2.5 ug/ml) (49). 
The high degree of resistance, the presence of both resistant 
and susceptible strains within the species, and their usually 
modest degree of pathogenicity make leuconostoc an excellent 
model for study of vancomycin resistance in the laboratory. 
Elucidation of the mechanism of resistance in these strains may 
add significantly to present knowledge of peptidoglycan 
synthesis, as well as the strategies involved in the development 
Recombinant DNA Research, Volume 13 
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