Non-Spore-Forming Bacteria

Most of the non-spore-forming species of bacteria isolated from insects are in the families Enterobacteriaceae or Pseudomonidaceae. Non-spore-forming bacteria generally have low pathogenicity when they occur in the digestive tract of an insect, but may be very pathogenic if they are able to enter the insect's hemocoel. Thus, diseases caused by non-spore-forming bacteria generally rely on a conditional factor to gain entrance into the hemocoel. The general term "stress" encompasses many of these conditions. Stress results from unusually high temperatures, poor food quality, crowding, mechanical injury, or other factors. Non-spore-forming bacteria that are not active invaders may enter the hemocoel when the insect has been stressed and/or injured. Once in the hemocoel, many non-spore-forming bacteria multiply rapidly and may cause death of the insect from bacterial septicemia within one or two days. The digestive tracts of most insects contain non-spore-forming bacteria capable of producing bacterial septicemia if they are able to get into the hemocoel. These are known collectively as facultative pathogens.

While bacterial septicemia caused by non-spore-forming bacteria occurs frequently in nature and may be an important natural mortality factor, it is very difficult to utilize non-spore-forming bacteria as biological control agents. They have limited invasive power, rarely cause conventional epizootics, and depend on specific conditions to gain entrance into the hemocoel and cause mortality. Bacterial septicemia is one of the most common diseases diagnosed in laboratory colonies of insects. Laboratory colonies are particularly prone to stress conditions that promote bacterial septicemia. In most cases, if the stress condition (extreme temperatures, poor food quality, crowding) is removed or prevented, bacterial septicemia can be eliminated as a cause of mortality.

Some non-spore-forming bacteria are pathogenic and can cause disease via ingestion of spores. One such bacterium is the Gram negative Serratia entomophila, a naturally occurring pathogen of grass grubs in New Zealand. Amber disease is characterized by cessation of feeding due attachment of the bacteria to the gut lining. The bacterium has been registered as a commercial product.  Serratia marcescens is another facultative pathogen of insects, causing septicemia. The non-spore-forming bacterium, Melissococcus pluton, is the causal agent of European foulbrood in honeybees.

The bacteria group also includes the Rickettsia, some species of which are pathogenic (particularly the Rickettsiella). Species of Wolbachia, previously considered nonpathogenic, have been implicated in reproductive incompatibility in many insect orders.

 Bacterial Pathogens | Spore-forming bacteria