Air-conditioner cooling towers as complex reservoirs and continuous source of Legionella pneumophila infection evidenced by a genomic analysis study in 2017, Switzerland

Introduction

Water supply and air-conditioner cooling towers (ACCT) are potential sources of Legionella pneumophila infection in people. During outbreaks, traditional typing methods cannot sufficiently segregate L. pneumophila strains to reliably trace back transmissions to these artificial water systems. Moreover, because multiple L. pneumophila strains may be present within these systems, methods to adequately distinguish strains are needed. Whole genome sequencing (WGS) and core genome multilocus sequence typing (cgMLST), with their higher resolution are helpful in this respect. In summer 2017, the health administration of the city of Basel detected an increase of L. pneumophila infections compared with previous months, signalling an outbreak.

Aim

We aimed to identify L. pneumophila strains populating suspected environmental sources of the outbreak, and to assess the relations between these strains and clinical outbreak strains.

Methods

An epidemiological and WGS-based microbiological investigation was performed, involving isolates from the local water supply and two ACCTs (n = 60), clinical outbreak and non-outbreak related isolates from 2017 (n = 8) and historic isolates from 2003–2016 (n = 26).

Results

In both ACCTs, multiple strains were found. Phylogenetic analysis of the ACCT isolates showed a diversity of a few hundred allelic differences in cgMLST. Furthermore, two isolates from one ACCT showed no allelic differences to three clinical isolates from 2017. Five clinical isolates collected in the Basel area in the last decade were also identical in cgMLST to recent isolates from the two ACCTs.

Conclusion

Current outbreak-related and historic isolates were linked to ACCTs, which form a complex environmental habitat where strains are conserved over years.

Enterotoxin Production of Bacillus thuringiensis Isolates From Biopesticides, Foods, and Outbreaks

While the relevance of Bacillus (B.) cereus as a major cause of gastroenteritis is undisputed, the role of the closely related B. thuringiensis in foodborne disease is unclear. B. thuringiensis strains frequently harbor enterotoxin genes. However, the organism has only very rarely been associated with foodborne outbreaks, possibly due to the fact that during outbreak investigations, B. cereus is routinely not differentiated from B. thuringiensis. A recent EFSA scientific opinion stresses the urgent need for further data allowing for improved risk assessment, in particular as B. thuringiensis is a commonly used biopesticide. Therefore, the aim of this study was to gain further insights into the hazardous potential of B. thuringiensis. To this end, 39 B. thuringiensis isolates obtained from commercially used biopesticides, various food sources, as well as from foodborne outbreaks were characterized by panC typing, panC-based SplitsTree analysis, toxin gene profiling, FTIR spectroscopic analysis, a cytotoxicity assay screening for enterotoxic activity, and a sphingomyelinase assay. The majority of the tested B. thuringiensis isolates exhibited low (23%, n = 9) or mid level enterotoxicity (74%, n = 29), and produced either no (59%, n = 23) or low levels (33%, n = 13) of sphingomyelinase, which is reported to act synergistically with enterotoxins Nhe and Hbl. One strain isolated from rosemary was however classified as highly enterotoxic surpassing the cytotoxic activity of the high-level reference strain by a factor of 1.5. This strain also produced vast amounts of sphingomyelinase. Combining all results obtained in this study into a fingerprint pattern, several enterotoxic biopesticide strains were indistinguishable from those of isolates from foods or collected in association with outbreaks. Our study shows that many B. thuringiensis biopesticide strains exhibit mid-level cytotoxicity in a Vero cell assay and that some of these strains cannot be differentiated from isolates collected from foods or in association with outbreaks. Thus, we demonstrate that the use of B. thuringiensis strains as biopesticides can represent a food safety risk, underpinning the importance of assessing the hazardous potential of each strain and formulation used.

[The role of common starlings (Sturnus vulgaris) in the epidemiology of bacterial, potentially human pathogenic, disease agents]

Since a long time a public garden in Basel is known as a site for overnight accommodation and assembly of starlings. The birds cause an immense faecal contamination of the park and the neighbouring district. A nursery and a primary school are directly affected. To evaluate the health risk coming from the starlings droppings for the population, particularly for the children and to assess the role of starlings in the transmission of diseases to humans and in the epidemiology of human diseases the presence of human bacterial pathogens in the faeces of starlings was determined. Some of the isolated strains were further typed and compared to strains of human origin. C. jejuni, L. monocytogenes and C. psittaci were most often found. The typing of some C. jejuni and L. monocytogenes isolates showed a great variety of geno-, sero- respectively phage types that did not belong to the strains most often found in isolates of human origin. Starlings can harbour human pathogens and therefore a potential risk of infection comes from their droppings. It seems however rather improbable, that these birds present a constant direct source of infection for human beings.

[Swarms of starlings in Basel: a natural phenomenon, a nuisance or a health risk?]

A Natural Phenomenon, Molestation or a Health Risk? For the last two decades an avenue in the centre of Basel (Altrheinweg, Klybeckquartier) is known as site for overnight accommodation and assembly of starlings (Sturnus vulgaris) during autumn. The nightly visitors cause an immense contamination that directly affects a nursery and a primary school. The aim of the present study was the evaluation of the health risk for the population due to the starlings' droppings, particularly for the children from the nursery school. Between August and December of the years 1995 and 1996 the population of starlings was observed, the contamination of the area with faeces was quantified, the presence of microorganisms in the faeces was determined and the contamination of the soil from the Altrheinweg was compared with that from other avenues and parks. Furthermore, the children attending the nursery school were monitored for possible infections due to the birds' droppings. In mid-October, when the population of starlings reached their maximum of about 15,000 birds, 1053 defecations per night and square metre were counted on an average in 1995 and 1821 in 1996. The microbiological analysis of the starlings' faeces showed a high contamination with Campylobacter spp. (1995: 95.5%; 1996: 100%), L. monocytogenes (1995: 36.5%; 1996: 26.7%) and C. psittaci (1995: 40.5%), a lower contamination with Salmonella spp. (1995: 1.0%), S. aureus (1995: 2.0%), Y. enterocolitica (1995: 1.5%) and Y. pseudotuberculosis (1995: 1.0%). Mycotic and parasitic human pathogens were not found. The contamination of the soil from the Altrheinweg with Campylobacter spp. and L. monocytogenes was significantly greater than that of the other avenues and parks in Basel. However, the investigations conducted by the school medical service showed that no infections among the children attending the nursery school could be correlated to the presence of the starlings. Even if the heavy contamination of the area with droppings of starlings presents a potential health risk, a continuous or systematic transmission of infectious agents seems rather improbable due to the contamination-avoiding behaviour of the children, the characteristics of the microorganisms involved and the climatic conditions in autumn. This situation showed however not reduce our awareness of such a potential health risk.

[The occurrence and significance of enterotoxin-producing Clostridium perfringens strains in the intestinal tract of horses]

100 faecal samples from clinically healthy horses of different age groups and feeding habits, 50 samples of faeces from horses suffering from enteropathy accompanied by diarrhoea and small and/or large intestine from 25 horses that had died after an intestinal disease were examined for the presence of Clostridium (Cl.) perfringens. The frequency with which Cl. perfringens was detected was 22% in clinically healthy horses, 32% in horses with diarrhoea and 52% in the dead horses. In two faecal samples from the horses with diarrhoea the microbial count of Cl. perfringens was ca. 10(6) cfu/g faeces. The occurrence of Cl. perfringens in clinically healthy horses was not influenced by age. In animals fed exclusively on grass silage Cl. perfringens was found more frequently. The enterotoxin of Cl. perfringens was identified in one of 36 faecal samples from horses with diarrhoea. 54 Cl. perfringens strains isolated from material examined were studied for their ability to sporulate and to produce enterotoxin. 98% of the strains formed spores. Enterotoxin production was negative in all cases. Experimental investigations with enterotoxin producing Cl. perfringens strains on four ponies did not show any signs of a possible durable colonization of the intestinal tract by this organism. The results of these investigations do not suggest any pathogenic relevance of enterotoxin producing Cl. perfringens strains to the development of enteropathy in horses.