Sensitivity of ant (Cephalotes) colonies and individuals to antibiotics implies feeding symbiosis with gut microorganisms

Ants in the tribe Cephalotini are exceptional in that they maintain microor ganisms in their digestive tract. To understand what these microorganisms m ean to the ants, we observed the feeding habits of Cephalotes pusillus and Cephalotes atratus, finding that in nature they feed on extrafloral nectars , homopteran secretions, and bird droppings. Feeding the antibiotic kanamyc in to colonies of C. pusillus in the laboratory kills them. Ants desiccate or starve rather than feed on liquids to which the antibiotics gentamycin a nd netilmycin have been added, but feed and survive on liquids containing n ystatin, penicillin, and ampicillin. We identified over 10 microorganisms f rom the intestine of C. pusillus with different antibiotic-resistance patte rns. The bacteria are from the genera Corynebacterium, Brevibacterium, Sphi ngobacterium, Ochrobactrum, Myroides, Brevundimonas, Alcaligenes, Stenotrop homonas, Moraxella, and Pseudomonas. We hypothesize that the microorganisms provide nutrients to the ants by synthesizing amino acids from carbohydrat es and nitrates. We do not know whether the ants collect the bacteria from the environment, but they transmit them to their young. They culture them i n their digestive tract, eventually feeding on them.