Spatial structure increases the benefits of antibiotic production in Streptomyces

Abstract

Bacteria in the soil compete for limited resources. One of the ways they might do this is by producing antibiotics, but the metabolic costs of antibiotics and their low concentrations have caused uncertainty about the ecological role of these products for the bacteria that produce them. Here, we examine the benefits of streptomycin production by the filamentous bacterium Streptomyces griseus. We first provide evidence that streptomycin production enables S. griseus to kill and invade the susceptible species, S. coelicolor, but not a streptomycin-resistant mutant of this species. Next, we show that the benefits of streptomycin production are density dependent, because production scales positively with cell number, and frequency dependent, with a threshold of invasion of S. griseus at around 1%. Finally, using serial transfer experiments where spatial structure is either maintained or destroyed, we show that spatial structure reduces the threshold frequency of invasion by more than 100-fold, indicating that antibiotic production can permit invasion from extreme rarity. Our results show that streptomycin is both an offensive and defensive weapon that facilitates invasion into occupied habitats and also protects against invasion by competitors. They also indicate that the benefits of antibiotic production rely on ecological interactions occurring at small local scales.

Keywords: Antibiotics; Streptomyces; microbial competition; spatial structure; streptomycin.

Figure 1

Competition between S. griseus and S. coelicolor mixed at equal frequencies. (A) Initial and final densities of S. griseus and a fully susceptible strain of S. coelicolor during four days of pairwise competition. (B) Fitness of S. griseus competed against susceptible S. coelicolor WT (MIC 2 µg/mL streptomycin), intermediate resistant (MIC 12, 24, or 48 µg/mL) or high‐level resistant (MIC 192 µg/mL) S. coelicolor mutants. Error bars represent standard errors of the mean

Figure 2

Quantification of S. griseus streptomycin production. (A) Halo size of streptomycin susceptible WT and resistant mutants of S. coelicolor when exposed to 10 mm agar plugs extracted from plates supplemented with purified streptomycin (final concentration 2–128 µg/mL streptomycin) or (B) from four‐day‐old plates inoculated with increasing densities of S. griseus spores. Error bars represent standard error of the mean

Figure 3

Competition between S. griseus and S. coelicolor at different densities or frequencies. (A) Fitness of S. griseus competing with an intermediate resistant S. coelicolor (MIC 48 µg/mL) at equal frequencies but varying spore densities (10²–10⁵) on the plate. (B) Fitness of S. griseus competing against S. coelicolor WT at a spore density of 10⁵ spores/plate from different frequencies (1%, 10%, and 50%) in the population. Error bars represent standard error of the mean

Figure 4

Changes in frequency of S. griseus in competition with streptomycin susceptible S. coelicolor WT when spatial structure is (A) maintained by replica plating or (B) destroyed by rotating the plate on the velvet used for replica plating in different directions. Inset in (A) shows a representative image of an inhibition zone around a S. griseus colony, scale bar 500 µm. (C) Representative images of plates from (A) and (B) at different initial S. griseus frequencies. (D) Frequency of S. griseus in competition with streptomycin susceptible or highly resistant S. coelicolor (MIC 192 µg/mL) when spatial structure is maintained