Honeybee-Specific Lactic Acid Bacterium Supplements Have No Effect on American Foulbrood-Infected Honeybee Colonies

Abstract

Paenibacillus larvae, the causative agent of American foulbrood (AFB), is the primary bacterial pathogen affecting honeybees and beekeeping. The main methods for controlling AFB are incineration of diseased colonies or prophylactic antibiotic treatment (e.g., with tylosin), neither of which is fully satisfactory. The search for superior means for controlling AFB has led to an increased interest in the natural relationships between the honeybee-pathogenic and mutualistic microorganisms and, in particular, the antagonistic effects of honeybee-specific lactic acid bacteria (hbs-LAB) against P. larvae These effects have been demonstrated only on individual larvae in controlled laboratory bioassays. Here we investigated whether supplemental administration of hbs-LAB had a similar beneficial effect on P. larvae infection at colony level. We compared experimentally AFB-infected colonies treated with hbs-LAB supplements to untreated and tylosin-treated colonies and recorded AFB symptoms, bacterial spore levels, and two measures of colony health. To account for the complexity of a bee colony, we focused on (Bayesian) probabilities and magnitudes of effect sizes. Tylosin reduced AFB disease symptoms but also had a negative effect on colony strength. The tylosin treatment did not, however, affect P. larvae spore levels and might therefore "mask" the potential for disease. hbs-LAB tended to reduce brood size in the short term but was unlikely to affect AFB symptoms or spores. These results do not contradict demonstrated antagonistic effects of hbs-LAB against P. larvae at the individual bee level but rather suggest that supplementary administration of hbs-LAB may not be the most effective way to harness these beneficial effects at the colony level.IMPORTANCE The previously demonstrated antagonistic effects of honeybee-derived bacterial microbiota on the infectivity and pathogenicity of P. larvae in laboratory bioassays have identified a possible new approach to AFB control. However, honeybee colonies are complex superorganisms where social immune defenses play a major role in resistance against disease at the colony level. Few studies have investigated the effect of beneficial microorganisms on bee diseases at the colony level. Effects observed at the individual bee level do not necessarily translate into similar effects at the colony level. This study partially fills this gap by showing that, unlike at the individual level, hbs-LAB supplements did not affect AFB symptoms at the colony level. The inference is that the mechanisms regulating the honeybee microbial dynamics within a colony are too strong to manipulate positively through supplemental feeding of live hbs-LAB and that new potential remedies identified through laboratory research have to be tested thoroughly in situ, in colonies.

Keywords: American foulbrood; Apis mellifera; Bifidobacterium; Lactobacillus; Paenibacillus larvae; enzootic disease; honeybee-specific lactic acid bacteria; honeybees; host-pathogen dynamics; tylosin.

FIG 1

Original data on the effect of tylosin and hbs-LAB on American foulbrood (AFB) and colony strength. Shown are AFB spore counts, clinical symptoms, and colony strength represented by colony size (number of frame sides with bees) and brood size (number of brood) with respect to treatment (control, American foulbrood infection; tylosin, American foulbrood infection and tylosin treatment; placebo, American foulbrood infection and placebo of hbs-LAB; hbs-LAB, American foulbrood infection and hbs-LAB). All six sampling occasions are shown for all 40 colonies, with the lines showing the trends between the mean values for the respective treatment and its control. At the bottom the time course of the treatment application is indicated. Red, tylosin; blue, = placebo/hbs-LAB; black, boost of infection by inoculating all colonies with AFB spores.

FIG 2

Model predictions of the effect of tylosin and hbs-LAB on American foulbrood and colony strength. Shown are predictions for a whole season (all six sampling occasions) (left side) and predictions for sampling right after the application of the hbs-LAB treatment (right side). The estimates and the difference show the mean value from 1,000 samples of the respective posterior with 89% credibility intervals (highest posterior density intervals). Each difference represents the posterior obtained by subtracting the posterior of one treatment by the posterior of the other treatment.