Editor’s note: Part one of “Microbes: It’s the Little Things That Matter” ran in the September 2016 issue of PCT. The author is a Board Certified Entomologist with the U.S. Navy Medical Service Corps, NCSU.
Even though Zika was certainly the buzzword of 2016, reports of bed bug infestations still manage to make the news on a regular basis, and the majority of PMPs still report beg bug treatment high on the list of calls. Schools, single-family homes, apartment buildings, roadside motels, commercial airlines and luxury hotels are not immune from this wide-spread problem. One of the drivers thought to be behind this profound resurgence is bed bugs’ resistance to many man-made insecticides that have been used extensively over the past two decades.
Bed bugs have several known mechanisms of insecticide resistance, and novel mechanisms have been uncovered recently. The common bed bug Cimex lectularius can hydrolyze (add water) to organophosphates, in other words, make the compound more water-soluble so that the active ingredient can then be excreted as waste more readily.
Many other insects, including bed bugs, have resistance mechanisms that use other enzymes called cytochrome P450s and glutathione S-transferases to degrade the active ingredient of several insecticides. Another mechanism found in bed bugs is known as a knockdown resistance (kdr) mutation. These are genetic changes that work to negate the intended effects of an insecticide that act on the insect nervous system, and thus provide resistance to pyrethroids and neonicotinoids. However, as recently as 2016, it was discovered that bed bugs from infestations treated continually with pyrethroids had thicker cuticles, thus making topical insecticides more difficult to penetrate.
EMERGING RESEARCH. Yet an even more fascinating, and still emerging field of research that blends entomology with microbiology, is exploring the role bacteria and other microorganisms that reside within insects have in detoxifying insecticides. This phenomenon has been found recently in agricultural pests such as stink bugs, moths and crickets.
Whether microbial-mediated insecticide resistance is occurring in bed bugs is essentially unknown at this time. In our present study, several species of bacteria known to degrade insecticides were discovered in the 200+ field-collected samples from across North Carolina we examined. These included bacteria previously shown in other studies to degrade pyrethroids commonly used against bed bugs, as well as organophosphates, such as parathion, chlorpyrifos, and lindane.
My project, under the guidance and expertise of Drs. Coby Schal and Wes Watson at North Carolina State University, sampled and identified all bacteria present in bed bug populations collected from non-transient housing at 15 locations around North Carolina.
With respect to bacteria that degrade insecticides, Pseudomonas was found at three locations separated by several hundred miles in one case. Pseudomonas fluorescens can rapidly degrade cypermethrin (Grant and Betts 2003, Grant et al. 2002) and permethrin (Maloney et al. 1988).Rhodanobacter
was found at seven locations and Stenotrophomonas was found at three locations. Rhodanobacter lindaniclasticus degrades lindane (Nalin et al. 1999), once common to lice shampoos, and Stenotrophomonas degrades both pyrethroids such as permethrin, bifenthrin and fenvalerate (Chen et al. 2011, Lee et al. 2004), and organophosphates such as methyl parathion, chlorpyrifos, diazinon, phoxim, triazophos, parathion (Deng et al. 2015).Brevibacterium
degrades cyfluthrin and parathion (Chen et al. 2013), and was found at two locations. Additionally, Methylibium, Agrobacterium, Lysinibacillus, and Brevundimonas diminuta were all found at four locations. All of these degrade organophosphate insecticides (Singh 2009, Kane et al. 2007, Liu et al. 2016).
The discovery of bacteria known to degrade insecticides within Cimex lectularius is remarkable, and may offer new evidence towards previously unknown mechanisms of insecticide resistance. Whether bed bugs are acquiring these bacteria as a fitness advantage is still undetermined, and warrants further attention since many species of bacteria can obtain nitrogen through the degradation of insecticides.