Growth of a water strain of Leptospira in synthetic media

A systematic review of Leptospira in water and soil environments

Background

Leptospirosis, caused by pathogenic Leptospira, is a zoonosis of global distribution. This infectious disease is mainly transmitted by indirect exposure to urine of asymptomatic animals via the environment. As human cases generally occur after heavy rain, an emerging hypothesis suggests that rainfall re-suspend leptospires together with soil particles. Bacteria are then carried to surface water, where humans get exposed. It is currently assumed that pathogenic leptospires can survive in the environment but do not multiply. However, little is known on their capacity to survive in a soil and freshwater environment.

Methods

We conducted a systematic review on Leptospira and leptospirosis in the environment in order to collect current knowledge on the lifestyle of Leptospira in soil and water. In total, 86 scientific articles retrieved from online databases or institutional libraries were included in this study.

Principals findings/significance

This work identified evidence of survival of Leptospira in the environment but major gaps remain about the survival of virulent species associated with human and animal diseases. Studies providing quantitative data on Leptospira in soil and water are a very recent trend, but must be interpreted with caution because of the uncertainty in the species identification. Several studies mentioned the presence of Leptospira in soils more frequently than in waters, supporting the hypothesis of the soil habitat and dispersion of Leptospira with re-suspended soil particles during heavy rain. In a near future, the growing use of high throughput sequencing will offer new opportunities to improve our understanding of the habitat of Leptospira in the environment. This better insight into the risk of leptospirosis will allow implementing efficient control measures and prevention for the human and animal populations exposed.

Growth requirements of pathogenic Leptospira

Nutritional requirements for growth at 30 C of Leptospira pomona and L. canicola have been determined. Both pathogenic serotypes initially required bovine serum albumin (BSA) for growth in a medium (SM-4) which permitted growth of the water isolate B-16. Requirement for BSA was eliminated by (i) removing much of the apparent toxicity of free fatty acids in Tween 80 on an anion exchange column, (ii) decreasing extended lag periods observed from small inocula by incorporation of pyruvate into the medium, (iii) the addition of acetate to permit full utilization of substrate fatty acids in Tween 80, and (iv) the addition of glycerol to decrease generation times. Physiologic significance of these findings is discussed, and the possibility is suggested that apparent toxicity of fatty acids for leptospires may result from their auto-oxidation products. The resulting protein-free medium (SM-5) permitted the growth of pathogens at 30 C to high cell yields in low inocula. Highly virulent and avirulent strains from the same clone of L. canicola Moulton were used to determine additional growth requirements associated with virulence. As incubation temperatures were increased from 30 C to those of mammalian hosts, virulent cells required biotin at 35 C and higher levels of K(+) and Mg(2+) at 37 C. Additional Fe(2+) eliminated the necessity for removing the toxicity of Tween 80 by anion exchange. Significance of these physiologic studies are discussed in relationship to virulence. The final protein-free medium (SM-6) grew highly virulent L. canicola from tissue to high yields from low inocula at 37 C with no loss in virulence over several transfers.

Shape of Treponema pallidum

Treponema pallidum was found to be not helical, but a flat wave twisted into one to five different planes per cell.