In vitro and in vivo studies of imipenem-cilastatin alone and in combination with gentamicin against Listeria monocytogenes

Human Listeriosis

Listeria monocytogenes is a Gram-positive facultative intracellular pathogen that can cause severe invasive infections upon ingestion with contaminated food. Clinically, listerial disease, or listeriosis, most often presents as bacteremia, meningitis or meningoencephalitis, and pregnancy-associated infections manifesting as miscarriage or neonatal sepsis. Invasive listeriosis is life-threatening and a main cause of foodborne illness leading to hospital admissions in Western countries. Sources of contamination can be identified through international surveillance systems for foodborne bacteria and strains' genetic data sharing. Large-scale whole genome studies have increased our knowledge on the diversity and evolution of L. monocytogenes, while recent pathophysiological investigations have improved our mechanistic understanding of listeriosis. In this article, we present an overview of human listeriosis with particular focus on relevant features of the causative bacterium, epidemiology, risk groups, pathogenesis, clinical manifestations, and treatment and prevention.

Antibiotic treatment for invasive nonpregnancy-associated listeriosis and mortality: a retrospective cohort study

Little evidence exists addressing the clinical value of adding gentamicin to ampicillin for invasive listeriosis. A multicenter retrospective observational study of nonpregnant adult patients with invasive listeriosis (primary bacteremia, central nervous system (CNS) disease, and others) in 11 hospitals in Israel between the years 2008 and 2014 was conducted. We evaluated the effect of penicillin-based monotherapy compared with early combination therapy with gentamicin, defined as treatment started within 48 h of culture results and continued for a minimum of 7 days. Patients who died within 48 h of the index culture were excluded. The primary outcome was 30-day all-cause mortality. A total of 190 patients with invasive listeriosis were included. Fifty-nine (30.6%) patients were treated with early combination therapy, 90 (46.6%) received monotherapy, and 44 (22.8%) received other treatments. Overall 30-day mortality was 20.5% (39/190). Factors associated with mortality included lower baseline functional status, congestive heart failure, and higher sequential organ failure assessment score. Source of infection, treatment type, and time from culture taken date to initiation of effective therapy were not associated with mortality. In multivariable analysis, monotherapy was not significantly associated with increased 30-day mortality compared with early combination therapy (OR 1.947, 95% CI 0.691-5.487). Results were similar in patients with CNS disease (OR 3.037, 95% CI 0.574-16.057) and primary bacteremia (OR 2.983, 95% CI 0.575-15.492). In our retrospective cohort, there was no statistically significant association between early combination therapy and 30-day mortality. A randomized controlled trial may be necessary to assess optimal treatment.

Listeria meningitis in an immunocompetent child

Listeria monocytogenes is a gram-positive, facultative anaerobic, motile bacillus widely spread in the environment. However, clinical infection in humans is uncommon. The route of infection is often through ingestion of raw vegetables, meat and unpasteurised dairy products. Listeria is an uncommon cause of meningitis in immunocompetent individuals, but it should be considered strongly in neonates and the elderly, and it should also be considered in those with impaired cell-mediated immunity. ^{1 , 2} Only a few case reports of invasive listeriosis in humans have been reported from India. ^{3 - 8} In our case, a 13-year-old previously healthy boy was thus diagnosed.

Antibiotic treatment and mortality in patients with Listeria monocytogenes meningitis or bacteraemia

Invasive Listeria monocytogenes infections carry a high mortality despite antibiotic treatment. The rareness of the infection makes it difficult to improve antibiotic treatment through randomized clinical trials. This observational study investigated clinical features and outcome of invasive L. monocytogenes infections including the efficacy of empiric and definitive antibiotic therapies. Demographic, clinical and biochemical findings, antibiotic treatment and 30-day mortality for all episodes of L. monocytogenes bacteraemia and/or meningitis were collected by retrospective medical record review in the North Denmark Region and the Capital Region of Denmark (17 hospitals) from 1997 to 2012. Risk factors for 30-day all-cause mortality were assessed by logistic regression. The study comprised 229 patients (median age: 71 years), 172 patients had bacteraemia, 24 patients had meningitis and 33 patients had both. Significant risk factors for 30-day mortality were septic shock (OR 3.0, 95% CI 1.4-6.4), altered mental state (OR 3.6, 95% CI 1.7-7.6) and inadequate empiric antibiotic therapy (OR 3.8, 95% CI 1.8-8.1). Cephalosporins accounted for 90% of inadequately treated cases. Adequate definitive antibiotic treatment was administered to 195 patients who survived the early period (benzylpenicillin 72, aminopenicillin 84, meropenem 28, sulfamethoxazole/trimethoprim 6, and piperacillin/tazobactam 5). Definitive antibiotic treatment with benzylpenicillin or aminopenicillin resulted in a lower 30-day mortality in an adjusted analysis compared with meropenem (OR 0.3; 95% CI 0.1-0.8). In conclusion, inadequate empiric antibiotic therapy and definitive therapy with meropenem were both associated with significantly higher 30-day mortality.

An update on the medical management of listeriosis

It is still not quite well understood why there is no optimal or even a satisfactory antibiotic therapy for listeriosis. Although almost all Listeria strains that induce sepsis, meningitis and encephalitis, as well as many other manifestations--in particular, in immunocompromised individuals--are susceptible to most of the common antibiotics, the cure rate is only approximately 70%. The most effective regimen still consists of a combination of an aminopenicillin (amoxicillin or ampicillin) plus an aminoglycoside. In vitro, this combination is bactericidal, whereas aminopenicillin alone only exerts a weak bactericidal activity against Listeriae. These antibiotics only poorly penetrate the cerebrospinal fluid and thus, only high doses given over a prolonged period of 2-3 weeks are curative. Furthermore, Listeria monocytogenes belongs to the group of facultative intracellular bacteria, which means that a certain population is inaccessible for antibiotics. Theoretically, a drug which is endowed with bactericidal activity superior to that of ampicillin would be preferable. Furthermore, the candidate drug should easily cross the blood-brain barrier into the CNS, be able to accumulate within host cells, reach the cytoplasm and be active under these unusual conditions. Because of all these arguments, the new quinolones are of particular interest; but broad clinical data are still lacking. It is unclear as to whether antibiotics alone will be sufficient to increase the prognosis. Adjunctive therapy with immunomodulators, which are able to reconstitute the defective defence capacities, would presumably create the conditions necessary to finally resolve listeriosis.

Fulminant Listeria monocytogenes meningitis complicated with acute hydrocephalus in healthy children beyond the newborn period

We describe 3 previously healthy Costa Rican children who had Listeria monocytogenes meningitis, an uncommon cause of bacterial meningitis beyond the newborn period in normal subjects. Two of them had initial normal brain computed tomography, but all 3 developed acute hydrocephalus at days 7, 3, and 5, respectively. All required immediate ventriculostomy placement and only 1 of 3 survived. L. monocytogenes should be considered among the etiologies of bacterial meningitis in children who do not respond initially to conventional antimicrobial treatment or who deteriorate rapidly.

Listeria monocytogenes-infected phagocytes can initiate central nervous system infection in mice

Listeria monocytogenes-infected phagocytes are present in the bloodstream of experimentally infected mice, but whether they play a role in central nervous system (CNS) invasion is unclear. To test whether bacteria within infected leukocytes could establish CNS infection, experimentally infected mice were treated with gentamicin delivered by surgically implanted osmotic pumps. Bacterial inhibitory titers in serum and plasma ranged from 1:16 to 1:256, and essentially all viable bacteria in the bloodstream of treated mice were leukocyte associated. Nevertheless, CNS infection developed in gentamicin-treated animals infected intraperitoneally or by gastric lavage, suggesting that intracellular bacteria could be responsible for neuroinvasion. This was supported by data showing that 43.5% of bacteria found with blood leukocytes were intracellular and some colocalized with F-actin, indicating productive intracellular parasitism. Experiments using an L. monocytogenes strain containing a chromosomal actA-gfpuv-plcB transcriptional fusion showed that blood leukocytes were associated with intracellular and extracellularly bound green fluorescent protein-expressing (GFP+) bacteria. Treatment with gentamicin decreased the numbers of extracellularly bound GFP+ bacteria significantly but did not affect the numbers of intracellular GFP+ bacteria, suggesting that the latter were the result of intercellular spread of GFP+ bacteria to leukocytes. These data demonstrate that infected leukocytes and the intracellular L. monocytogenes harbored within them play key roles in neuroinvasion. Moreover, they suggest that phagocytes recruited to infected organs such as the liver or spleen are themselves parasitized by intercellular spread of L. monocytogenes and then reenter the bloodstream and contribute to the systemic dissemination of bacteria.

Management of listeriosis

Determination of the MIC in vitro is often used as the basis for predicting the clinical efficacy of antibiotics. Listeriae are uniformly susceptible in vitro to most common antibiotics except cephalosporins and fosfomycin. However, the clinical outcome is poor. This is partially because listeriae are refractory to the bactericidal mechanisms of many antibiotics, especially to ampicillin-amoxicillin, which still is regarded as the drug of choice. A true synergism can be achieved by adding gentamicin. Another point is that listeriae are able to reside and multiply within host cells, e.g., macrophages, hepatocytes, and neurons, where they are protected from antibiotics in the extracellular fluid. Only a few agents penetrate, accumulate, and reach the cytosol of host cells, where the listeriae are found. Furthermore, certain host cells may exclude antibiotics from any intracellular compartment. Thus, determination of the antibacterial efficacy of a drug against listeriae in cell cultures may be a better approximation of potential therapeutic value. Certain host cells may have acquired the property of excluding certain antibiotics, for example macrolides, from intracellular spaces, which might explain therapeutic failures of antibiotic therapy in spite of low MICs. Animal models do not completely imitate human listeriosis, which is characterized by meningitis, encephalitis, soft tissue and parenchymal infections, and bacteremia. Meningitis produced in rabbits is a hyperacute disease, whereby most listeriae lie extracellularly, fairly accessible to antibiotics that can cross the blood-cerebrospinal fluid barrier. In the murine model of systemic infection, Listeria monocytogenes is located mainly within macrophages and parenchymal cells of the spleen and liver, hardly accessible to certain drugs, such as ampicillin and gentimicin. The therapeutic efficacy of drugs clearly depends on the model used. Thus, for example, the combination of ampicillin with gentamicin acts synergistically in the rabbit meningitis model but not in the mouse model. Since conventional antimicrobial therapy with antibiotics is not satisfactory, particularly in the immunocompromised host (about 30% of patients with listeriosis die in spite of a rational choice of antibiotics), other possibilities must be considered for therapy as well as prevention. Indeed, listeriae are highly susceptible to several endogenous antibiotics, such as defensins. Bacteriocins produced by related bacterial species, e.g., lactobacilli and enterococci, are rapidly bactericidal. However, unfortunately, the use of such alternative measures along with immunization and immunmodulation is not yet feasible.

Antimicrobial chemotherapy of human infection due to Listeria monocytogenes

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.

Inhibition of intracellular growth of Listeria monocytogenes by antibiotics

We studied the activities of 15 antibiotics on the intracellular growth of Listeria monocytogenes in a HeLa cell line. After 24 h of contact with the infected cells, the antibiotics most effective against the intracellular growth of the 10 strains tested were amoxicillin, temafloxacin, and sparfloxacin, which nevertheless failed to totally eliminate the intracellular bacteria. Rifampin and co-trimoxazole had variable effects, depending on the isolates studied. The most active combinations were amoxicillin-sparfloxacin, co-trimoxazole-gentamicin, and sparfloxacin-co-trimoxazole. The results suggest the value of using a cell culture technique to study the activities of antibiotics against certain bacteria with intracellular sites of multiplication.

Imipenem/cilastatin. A review of its antibacterial activity, pharmacokinetic properties and therapeutic efficacy

Imipenem is the first available semisynthetic thienamycin and is administered intravenously in combination with cilastatin, a renal dipeptidase inhibitor that increases urinary excretion of active drug. In vitro studies have demonstrated that imipenem has an extremely wide spectrum of antibacterial activity against Gram-negative and Gram-positive aerobic and anaerobic bacteria, even against many multiresistant strains of bacteria. It is very potent against species which elaborate beta-lactamases. Imipenem in combination with equal doses of cilastatin has been shown to be generally well tolerated and an effective antimicrobial for the treatment of infections of various body systems. It is likely to be most valuable as empirical treatment of mixed aerobic and anaerobic infections, bacteraemia in non-neutropenic patients and serious hospital-acquired infections.