The genetic basis of tetracycline resistance was studied in 25 clinical isolates of Listeria monocytogenes. Resistance to tetracycline was associated with resistance to minocycline and due to the presence of the tet(M) gene in 24 strains. Association of tet(M) with int-Tn, the gene encoding the protein required for the movements of Tn1545-like conjugative transposons, was found in all strains. Cotransfer of tet(M) and int-Tn among L. monocytogenes cells and from L. monocytogenes to Enterococcusfaecalis was detected in 7 of the 12 strains studied at frequencies similar to those obtained with the prototype element Tnl545. tet(L), the secondmost prevalent tetracycline resistance gene in enterococci and streptococci, was detected in the remaining strain, where it was borne by a 5-kb plasmid. These observations indicate that two types of movable genetic elements, transposons and plasmids, in enterococci and streptococci are responsible for emergence of drug resistance in L. monocytogenes.

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Listeriosis is an uncommon infection, but when it occurs it carries a high mortality rate. Early diagnosis is essential and thereafter appropriate antimicrobial chemotherapy. Ampicillin or penicillin plus gentamicin remains the treatment of choice for most manifestations of listeriosis, and adequate doses must be given, i.e. greater than 6g/day of ampicillin or penicillin. Co-trimoxazole appears to be an excellent alternative agent with good penetration into the cerebrospinal fluid. Vancomycin is an appropriate agent for the treatment of primary bacteraemia but does not cross the blood-brain barrier sufficiently well to be useful in meningitis, while erythromycin may be used to treat listeriosis in cases of pregnancy. Treatment of bacteraemia requires one to two weeks' therapy, while meningitis cases may need to be treated for longer; for example, it has been found that most patients with acute meningitis in the UK were treated for 20 days. Infective endocarditis needs treatment for six to eight weeks. Doses should be varied with patients' altered organ function and antimicrobial serum monitoring performed when appropriate.

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Summary. The virulence of representative strains of the five species of Listeria rnonocytogenes sensu lato was compared in C57BL/6 and BALB/c mice in terms of LD50 values and of bacterial growth kinetics and histological changes in mouse livers. L. monocytogenes sensu stricto and L. ivanovii showed relatively low LD50 values and much bacterial growth for 2-3 days before viable counts declined. Histological changes in L. ivanovii infection resembled those caused by L. rnonocytogenes, with early development of neutrophil-rich micro-abscesses and hepatocyte necrosis followed by macrophage infiltration and formation of granulomas. By contrast, L. innocua, L. welshirneri and L. seeligeri were almost entirely avirulent as shown by high LD50 values, early elimination of viable bacteria and no evidence of growth. Histological changes consisted only of slight, transient infiltration of the liver with neutrophils. Both groups of bacteria were seen infrequently in Kupffer cells early in infection, but only the highly virulent species appeared to replicate. LD50 values for L. rnonocytogenes and L. ivanovii were (10-20)-fold greater, and for the less virulent bacteria at least two-fold greater, in C57BL/6 than in BALB/c mice. This difference in host susceptibility was not reflected in detectable histological differences between the two mouse strains.