Stimulation of aquatic bacteria from Mammoth Cave, Kentucky, by sublethal concentrations of antibiotics

Thomas D. Byl, Petra K. Byl, Jacob Byl, and Rickard Toomey III


Many microorganisms secrete secondary metabolites with antibiotic properties; however, there is debate whether the secretions evolved as a means to gain a competitive edge or as a chemical signal to coordinate community growth. The objective of this research was to investigate if select antibiotics acted as a weapon or as a chemical signal by exposing communities of aquatic cave bacteria to increasing concentrations of antibiotics. Water samples were collected from six cave locations where actinobacterial mats appeared to be plentiful. Bacterial growth was measured using colony counts on 10 % tryptic soy agar augmented with increasing concentrations of erythromycin, tetracycline, kanamycin, gentamicin, or quaternary ammonia compounds (QAC). Colony counts generally decreased as the gentamicin, kana mycin and QAC dose increased. In contrast, the colony numbers increased on agar plates supplemented with 0.01 mg L?1, 0.10 mg L?1 and 1.00 mg L?1 erythromycin or tetracycline. A 10.00 mg L?1 dose of each antibiotic treatment reduced bacteria colonies by 98 % or more. Community-level physiological capabilities were evaluated using Ecolog plates inoculated with cave water dosed with either 0.00 mg L?1 or 0.10 mg L?1 of erythromycin. Incubation with the antibiotic almost doubled the number of food substrates used in the first 24 hours. There was a significant increase in the use of acetyl glucosamine, arginine, and putrescine when bacteria were exposed to 0.10 mg L?1 erythromycin triggered by the antibiotic acting as a chemical messenger. Principal component analysis confirmed a shift in substrate preferences when erythromycin was added. A conceptual ecological model is proposed based on the response of aquatic cave bacteria to sublethal antibiotics.


Many tiny organisms produce substances with antibiotic properties. However, scientists aren't sure if these substances are meant to give the organisms a competitive edge over others, or if they serve as chemical signals to help coordinate their growth. This study aimed to figure out whether these antibiotics act as weapons or signals in communities of bacteria that live in water caves. The researchers collected water samples from six caves where certain types of bacteria seemed to be thriving. They then measured the growth of these bacteria on a nutrient-rich substance, where they increased the amounts of various antibiotics. As the dose of some antibiotics (gentamicin, kanamycin, and a group of compounds known as QAC) increased, the number of bacterial colonies generally decreased. However, with two antibiotics, erythromycin and tetracycline, something different happened. The number of bacterial colonies actually increased when small amounts of these antibiotics were added. But when a much larger dose was added, almost all the bacteria were killed. The researchers also tested how the bacteria's ability to use different food sources changed when exposed to erythromycin. They found that the bacteria were able to use almost twice as many types of food within the first day of exposure to the antibiotic. Certain types of food were particularly more popular when the bacteria were exposed to this antibiotic. This suggests that the antibiotic might be acting as a chemical messenger. The researchers concluded that the bacteria's reaction to the antibiotics suggests a shift in how they use their food sources when exposed to erythromycin. This leads them to propose a model to explain how these water cave bacteria react to antibiotics.