Serodiagnosis and Immunotherapy in Infectious Disease (1989) 3, 167-l 73 Enhanced binding of murine monoclonal antibodies to lipopolysaccharide structures of Enterobacteriaceae after treatment with antibiotics Berry P. Overbeek*, N. Macbiel de Vos, Nathan Keller, Giammarco Raponi, Maja Rozenberg-Arska and Jan Verhoef Laboratory for Microbiology, Department of Medical Microbiology, State University of Utrecht. Utrecht. The Netherlands Four strains of E. coli, two encapsulated and two unencapsulated, weregrown in the presence or absence of the g-lactamantibiotics aztreonam,ceftazidime and ceftriax- one, the fluoroquinolone ciprofloxacin, and the aminoglycoside netilmicin. Treat- ment of the unencapsulated strains with g-lactamsand ciprofloxacin resulted in enhanced binding of murine Mabs (all:IgM) directed to rough lipopolysaccharide structures in ELBA. However, binding of these Mabs to antibiotic treated E. coli 07K1 (bearinga thin capsule) wasonly slightly increased, and wasunchanged in E. cofi 08K49 (surrounded by a thick capsule). Treatment of bacteriawith netilmicindid not result in differences in binding of these monoclonal antibodies. These results show that the inner parts of the lipopolysaccharide become more accessible to antibodies when, especially unencapsulated, bacteria are grown in the presence of certain antibiotics. Keywords: Beta-lactam antibiotics, ciprofloxacin, Escherichia co/i, gram-negative bacteria, membrane, outer, sub-inhibitory concentration. Introduction In 1968 Chedid et al. were the first to suggest that shared antigens, conserved in the core portion of lipopolysacchatide (LPS), might be responsible for the induction of antibod- ies that protect not only against homologous, but against heterologous bacterial infections as well’. Polyclonal antisera, raised against heat-killed rough (R) mutants of Escherichia colt’ or Salmonella minnesota were shown to react with heterologous gram- negative bacteriazm5. Murine and human monoclonal antibodies (Mabs) raised to the J5 mutant, chemotype Rc, of Escherichiu coli 0111 also have been shown to cross-react with heterologous gram-negative bacteria &a However, other investigators were not able . to demonstrate cross-reactivity with such Mabs9*“. Recently, Calandra et al. reported that intravenous treatment with human IgG antibodies to E. co/i J5 LPS did not reduce the mortality in patients with gram-negative septic shock, when compared with treatment with a standard IgG preparation”. This could be explained by the findings of McCabe et al., who reported that the protective activity of antiserum, raised in humans or rabbits against the Re mutant of *Address for correspondence: Kootwijkerschans 1,3432 CX Nieuwegein, TheNetherlands. 167 0888-0786/89/030167+07 $03.00/O 0 1989 AcademicPress Limited