Stephen B. Calderwood

Acute bacterial meningitis in adults. A review of 493 episodes.

BACKGROUND AND METHODS To characterize acute bacterial meningitis in adults, we reviewed the charts of all persons 16 years of age or older in whom acute bacterial meningitis was diagnosed at Massachusetts General Hospital from 1962 through 1988. We included patients who were admitted after initial treatment at other hospitals. RESULTS During the 27-year period, 445 adults were treated for 493 episodes of acute bacterial meningitis, of which 197 (40 percent) were nosocomial. Gram-negative bacilli (other than Haemophilus influenzae) caused 33 percent of the nosocomial episodes but only 3 percent of the community-acquired episodes. In the 296 episodes of community-acquired meningitis, the most common pathogens were Streptococcus pneumoniae (37 percent), Neisseria meningitidis (13 percent), and Listeria monocytogenes (10 percent); these organisms accounted for only 8 percent of the nosocomial episodes. Only 19 of the 493 episodes of meningitis (4 percent) were due to H. influenzae. Nine percent of all patients had recurrent meningitis; many had a cerebrospinal fluid leak. Seizures occurred in 23 percent of patients with community-acquired meningitis, and 28 percent had focal central nervous system findings. Risk factors for death among those with single episodes of community-acquired meningitis included older age (> or = 60 years), obtunded mental state on admission, and seizures within the first 24 hours. Among those with single episodes, the in-hospital mortality rate was 25 percent for community-acquired and 35 percent for nosocomial meningitis. The overall case fatality rate was 25 percent and did not vary significantly over the 27 years. CONCLUSIONS In our large urban hospital, a major proportion of cases of acute bacterial meningitis in adults were nosocomial. Recurrent episodes of meningitis were frequent. The overall mortality rate remained high.

Role of iron in regulation of virulence genes.

The abilities of bacterial pathogens to adapt to the environment within the host are essential to their virulence. Microorganisms have adapted to the iron limitation present in mammalian hosts by evolving diverse mechanisms for the assimilation of iron sufficient for growth. In addition, many bacterial pathogens have used the low concentration of iron present in the host as an important signal to enhance the expression of a wide variety of bacterial toxins and other virulence determinants. The molecular basis of coordinate regulation by iron has been most thoroughly studied in Escherichia coli. In this organism, coordinate regulation of gene expression by iron depends on the regulatory gene, fur. Regulation of gene expression by iron in a number of pathogenic organisms is coordinated by proteins homologous to the Fur protein of E. coli. Additional regulatory proteins may be superimposed on the Fur repressor to provide the fine-tuning necessary for the precise regulation of individual virulence genes in response to iron and other environmental signals. Studies of the mechanisms of regulation of iron acquisition systems and virulence determinants by iron should lead to a better understanding of the adaptive response of bacteria to the low-iron environment of the host and its importance in virulence.

The Origin of the Haitian Cholera Outbreak Strain

BACKGROUND Although cholera has been present in Latin America since 1991, it had not been epidemic in Haiti for at least 100 years. Recently, however, there has been a severe outbreak of cholera in Haiti. METHODS We used third-generation single-molecule real-time DNA sequencing to determine the genome sequences of 2 clinical Vibrio cholerae isolates from the current outbreak in Haiti, 1 strain that caused cholera in Latin America in 1991, and 2 strains isolated in South Asia in 2002 and 2008. Using primary sequence data, we compared the genomes of these 5 strains and a set of previously obtained partial genomic sequences of 23 diverse strains of V. cholerae to assess the likely origin of the cholera outbreak in Haiti. RESULTS Both single-nucleotide variations and the presence and structure of hypervariable chromosomal elements indicate that there is a close relationship between the Haitian isolates and variant V. cholerae El Tor O1 strains isolated in Bangladesh in 2002 and 2008. In contrast, analysis of genomic variation of the Haitian isolates reveals a more distant relationship with circulating South American isolates. CONCLUSIONS The Haitian epidemic is probably the result of the introduction, through human activity, of a V. cholerae strain from a distant geographic source. (Funded by the National Institute of Allergy and Infectious Diseases and the Howard Hughes Medical Institute.).

Host-induced epidemic spread of the cholera bacterium

The factors that enhance the transmission of pathogens during epidemic spread are ill defined. Water-borne spread of the diarrhoeal disease cholera occurs rapidly in nature, whereas infection of human volunteers with bacteria grown in vitro is difficult in the absence of stomach acid buffering. It is unclear, however, whether stomach acidity is a principal factor contributing to epidemic spread. Here we report that characterization of Vibrio cholerae from human stools supports a model whereby human colonization creates a hyperinfectious bacterial state that is maintained after dissemination and that may contribute to epidemic spread of cholera. Transcriptional profiling of V. cholerae from stool samples revealed a unique physiological and behavioural state characterized by high expression levels of genes required for nutrient acquisition and motility, and low expression levels of genes required for bacterial chemotaxis.

A simple model host for identifying Gram-positive virulence factors

We demonstrate the use of the nematode Caenorhabditis elegans as a facile and inexpensive model host for several Gram-positive human bacterial pathogens. Enterococcus faecalis, Streptococcus pneumoniae, and Staphylococcus aureus, but not Bacillus subtilis, Enterococcus faecium, or Streptococcus pyogenes, kill adult C. elegans. Focusing our studies on the enterococcal species, we found that both E. faecalis and E. faecium kill C. elegans eggs and hatchlings, although only E. faecalis kills the adults. In the case of adults, a low inoculum of E. faecalis grows to a high titer in the C. elegans intestine, resulting in a persistent infection that cannot be eradicated by prolonged feeding on E. faecium. Interestingly, a high titer of E. faecium also accumulates in the nematode gut, but does not affect the longevity of the worms. Two E. faecalis virulence-related factors that play an important role in mammalian models of infection, fsr, a putative quorum-sensing system, and cytolysin, are also important for nematode killing. We exploit the apparent parallels between Gram-positive infection in simple and more complex organisms by using the nematode to identify an E. faecalis virulence factor, ScrB, which is relevant to mammalian pathogenesis.

Central nervous system infection with Listeria monocytogenes. 33 years' experience at a general hospital and review of 776 episodes from the literature.

We reviewed 776 previously reported and 44 new cases of CNS listeriosis outside of pregnancy and the neonatal period, and evaluated the epidemiologic, diagnostic, and therapeutic characteristics of this infection. Among patients with Listeria meningitis/meningoencephalitis, hematologic malignancy and kidney transplantation were the leading predisposing factors, but 36% of patients had no underlying diseases recognized. The infection occurred throughout life, with a higher incidence before the age of 3 and after the age of 45-50 years. Fever, altered sensorium, and headache were the most common symptoms, but 42% of patients had no meningeal signs on admission. Compared with patients with acute meningitis due to other bacterial pathogens, patients with Listeria infection had a significantly lower incidence of meningeal signs, and the CSF profile was significantly less likely to have a high WBC count or a high protein concentration. Gram stain of CSF was negative in two-thirds of cases of CNS listeriosis. One-third of patients had focal neurologic findings, and approximately one-fourth developed seizures over their course. Mortality was 26% overall, and was higher among patients with seizures and those older than 65 years of age. Relapse occurred in 7% of episodes. Ampicillin for a minimum of 15-21 days (with an aminoglycoside for at least the first 7-10 days) remains the treatment of choice. Cerebritis/abscess due to L. monocytogenes, without meningeal involvement, is less common but may be diagnosed by blood cultures and CNS imaging, or by stereotactic biopsy. Longer antibiotic therapy (at least 5-6 weeks) is needed in the presence of localized CNS involvement.

Galleria mellonella as a Model System To Study Cryptococcus neoformans Pathogenesis

ABSTRACT Evaluation of Cryptococcus neoformans virulence in a number of nonmammalian hosts suggests that C. neoformans is a nonspecific pathogen. We used the killing of Galleria mellonella (the greater wax moth) caterpillar by C. neoformans to develop an invertebrate host model system that can be used to study cryptococcal virulence, host immune responses to infection, and the effects of antifungal compounds. All varieties of C. neoformans killed G. mellonella. After injection into the insect hemocoel, C. neoformans proliferated and, despite successful phagocytosis by host hemocytes, killed caterpillars both at 37°C and 30°C. The rate and extent of killing depended on the cryptococcal strain and the number of fungal cells injected. The sequenced C. neoformans clinical strain H99 was the most virulent of the strains tested and killed caterpillars with inocula as low as 20 CFU/caterpillar. Several C. neoformans genes previously shown to be involved in mammalian virulence (CAP59, GPA1, RAS1, and PKA1) also played a role in G. mellonella killing. Combination antifungal therapy (amphotericin B plus flucytosine) administered before or after inoculation was more effective than monotherapy in prolonging survival and in decreasing the tissue burden of cryptococci in the hemocoel. The G. mellonella-C. neoformans pathogenicity model may be a substitute for mammalian models of infection with C. neoformans and may facilitate the in vivo study of fungal virulence and efficacy of antifungal therapies.

Killing of Caenorhabditis elegans by Cryptococcus neoformans as a model of yeast pathogenesis

We found that the well-studied nematode Caenorhabditis elegans can use various yeasts, including Cryptococcus laurentii and Cryptococcus kuetzingii, as a sole source of food, producing similar brood sizes compared with growth on its usual laboratory food source Escherichia coli OP50. C. elegans grown on these yeasts had a life span similar to (C. laurentii) or longer than (C. kuetzingii) those fed on E. coli. However, the human pathogenic yeast Cryptococcus neoformans killed C. elegans, and the C. neoformans polysaccharide capsule as well as several C. neoformans genes previously shown to be involved in mammalian virulence were also shown to play a role in C. elegans killing. These included genes associated with signal transduction pathways (GPA1, PKA1, PKR1, and RAS1), laccase production (LAC1), and the α mating type. C. neoformans adenine auxotrophs, which are less virulent in mammals, were also less virulent in C. elegans. These results support the model that mammalian pathogenesis of C. neoformans may be a consequence of adaptations that have evolved during the interaction of C. neoformans with environmental predators such as free-living nematodes and amoebae and suggest that C. elegans can be used as a simple model host in which C. neoformans pathogenesis can be readily studied.

Cholera transmission: the host, pathogen and bacteriophage dynamic

Zimbabwe offers the most recent example of the tragedy that befalls a country and its people when cholera strikes. The 2008–2009 outbreak rapidly spread across every province and brought rates of mortality similar to those witnessed as a consequence of cholera infections a hundred years ago. In this Review we highlight the advances that will help to unravel how interactions between the host, the bacterial pathogen and the lytic bacteriophage might propel and quench cholera outbreaks in endemic settings and in emergent epidemic regions such as Zimbabwe.

Caenorhabditis elegans as a Model Host for Staphylococcus aureus Pathogenesis

ABSTRACT Staphylococcus aureus, an important pathogen of humans and other warm-blooded animals, is also capable of killing the nematode Caenorhabditis elegans. Here, we show that C. elegans organisms that are fed S. aureus die over the course of several days in a process that is correlated with the accumulation of bacteria within the nematode digestive tract. Several S. aureus virulence determinants known or speculated to be important in mammalian pathogenesis, including the quorum-sensing global virulence regulatory system agr and the global virulence regulator sarA, the alternative sigma factor σB, alpha-hemolysin, and V8 serine protease, are required for full pathogenicity in nematodes. In addition, several defined C. elegans mutants were examined for susceptibility to S. aureus infection. Enhanced susceptibility to S. aureus killing was observed with loss-of-function mutations in the C. elegans genes esp-2/sek-1 and esp-8/nsy-1, which encode components of a conserved p38 MAP kinase signaling pathway involved in nematode defense against multiple pathogens. These results suggest that key aspects of S. aureus pathogenesis have been conserved, irrespective of the host, and that specific C. elegans host factors can alter susceptibility to this gram-positive human pathogen.