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Wolbachia is a genus of Gram-negative bacteria that live inside the cells of many arthropods and some nematodes. They belong to the family Anaplasmataceae in the order Rickettsiales of the class Alphaproteobacteria, which also includes the agents of typhus and Rocky Mountain spotted fever, and they are obligate intracellular symbionts that cannot survive outside host cells.

Wolbachia is among the most widespread bacterial genera on Earth. Surveys suggest that a large fraction of insect species carry it, with published estimates ranging from roughly a fifth to around two thirds depending on the groups sampled and the detection methods used. Well-studied groups are over-represented in these surveys, so the true figure is uncertain. It also infects mites, spiders, crustaceans, and the filarial worms that cause onchocerciasis (river blindness) and lymphatic filariasis.

The bacteria are passed from mother to offspring in the egg cytoplasm, a mode of inheritance called vertical transmission, and they are almost never transmitted by males. This pattern has driven the evolution of a set of manipulations of host reproduction, all of which increase the proportion of infected females in a population. In some hosts, by contrast, Wolbachia has become a mutualist that the host cannot live without.

Because Wolbachia can spread through a population by itself and interferes with the replication of several human pathogens, it has become one of the most widely deployed tools in modern vector control, used against dengue fever and related viruses in more than a dozen countries.

Discovery

Wolbachia was first seen in 1924 by Marshall Hertig and Simeon Burt Wolbach, who described rod-shaped bacteria in the reproductive tissues of the mosquito Culex pipiens. Hertig formally named the organism Wolbachia pipientis in 1936, after his colleague. For several decades it was regarded as a curiosity. Interest grew in the 1950s and 1960s, when crosses between Culex populations from different regions were found to fail in a pattern that was later matched to the presence of these bacteria. Hannes Laven studied this incompatibility in the 1950s and 1960s, and in 1967 he reported the local elimination of Culex quinquefasciatus from a village in Myanmar by the release of incompatible males. Interest rose again in the 1990s, when PCR-based surveys revealed how common the bacteria are.

Taxonomy and supergroups

Formally the genus Wolbachia contains the single named species Wolbachia pipientis, but this is a convenience and not a biological statement. The group contains deeply divergent lineages, and the naming is likely to change. Strains are instead assigned to supergroups, which are designated by letters:

• Supergroups A and B contain most arthropod strains, including those used in mosquito control.
• Supergroups C and D contain the strains found in filarial nematodes, where the relationship is mutualistic.
• Supergroup F is unusual in containing strains from both arthropods and nematodes.
• Further supergroups have been proposed from arthropods, from plant-parasitic nematodes, and from other hosts.

Individual strains are conventionally named with a lower-case w followed by an abbreviation of the host. The strain _w_Mel comes from Drosophila melanogaster, _w_AlbB from Aedes albopictus, _w_Pip from Culex pipiens, and _w_Cle from the bedbug Cimex lectularius.

Genome

Wolbachia genomes are small, generally between about 0.9 and 1.5 million base pairs. They show the pattern typical of long-term intracellular bacteria: many genes have been lost, particularly those for making amino acids and other compounds that the host can supply. The genomes of mutualistic strains tend to be smaller and more stable than those of arthropod strains, which carry many mobile genetic elements, repeated sequences, and prophages.

The most important of these is phage WO, a bacteriophage that is integrated into the genomes of most arthropod strains. It carries the genes that are responsible for cytoplasmic incompatibility, the best-understood of the reproductive manipulations, and it provides a route by which genes move between Wolbachia strains that share a host.

Cell biology

Wolbachia lives within a host-derived membrane-bound vacuole. It is concentrated in the reproductive tissues, and in many hosts it is also found in the gut, fat body, muscle, and nervous tissue. In the germline, the bacteria use the host cytoskeleton, particularly microtubules and the motor protein kinesin, to reach the posterior pole of the developing oocyte, where they are incorporated into the cells that give rise to the next generation. The density of bacteria in a host is regulated by the strain, by the host genotype, and by temperature, and it influences both the strength of reproductive manipulation and the degree of virus blocking.

Transmission

Wolbachia is inherited through the egg. Bacteria concentrate in the germline of the female and are packaged into developing oocytes. Sperm are too small to carry a meaningful bacterial population, so infected males are an evolutionary dead end. Transmission is usually efficient but rarely perfect, and a small proportion of the offspring may escape infection. Heat and some other stresses reduce the efficiency of transmission.

Over longer timescales, Wolbachia also moves horizontally between host species, as shown by the frequent mismatch between bacterial and host phylogenies. The routes are mostly inferred and not observed. Shared parasitoid wasps, predation, and feeding on the same plant have all been proposed.

A further consequence of this intimacy is that fragments of the Wolbachia genome, and occasionally almost the whole of it, have been transferred into the chromosomes of host animals by horizontal gene transfer. Such insertions are known from many insects and nematodes. They are usually non-functional, and their main practical importance is that they can produce false positives in surveys for live infection.

Because only females transmit Wolbachia, any variant that increases the number or the proportion of infected daughters will spread, even if it harms the host overall. Four such manipulations are known. From the point of view of the host, all of them are forms of reproductive parasitism.

Reproductive manipulations by Wolbachia

Cytoplasmic incompatibility

The most common and best-studied manipulation by Wolbachia is cytoplasmic incompatibility (CI). A cross between an infected male and an uninfected female produces few or no offspring. Every other combination is normal, including an infected male with an infected female that carries the same strain, in which the incompatibility is reversed ("rescued"). Where two different strains are involved, incompatibility can run in both directions (bidirectional CI), which is the basis of some population suppression methods.

The failure occurs at the first division of the fertilized egg. The paternal chromosomes are modified in the sperm, condense late, and are not properly incorporated, which leads to abnormal division and death of the embryo. In haplodiploid hosts, the same defect can produce males from fertilized eggs and not death.

Two genes carried by phage WO, usually designated cifA and cifB and found next to each other, are responsible. Expressing them together in uninfected male flies reproduces the sterility, and expressing cifA in females rescues it. Their products come in several biochemical varieties, and they include a deubiquitylase (cid type) and nucleases (cin type). How these genes work is not settled, and two models are debated:

• The toxin-antidote model treats cifB as a toxin that is deposited in sperm, and cifA in the egg as its antidote. Infected females provide the antidote, and uninfected ones do not.
• The host modification model holds that the two products act together in the developing sperm to modify it, in such a way that only maternally supplied cifA can later reverse the change.

The distinction matters for interpreting experiments and for predicting how readily hosts or bacteria might evolve around the system, but both accounts explain the basic pattern of crosses. Some strains of Wolbachia have lost the ability to induce CI, and the strength of CI depends on the strain, the age of the male, and the density of bacteria.

Spread through a population

In _Wolbachia_-infected populations, cytoplasmic incompatibility creates an unusual dynamic. When the infection is rare, almost every male that a female meets is uninfected, so being infected confers little advantage, whereas carrying bacteria may impose a cost. When the infection is common, uninfected females frequently mate with infected males and lose their broods, so infection is strongly favored.

The result is an unstable equilibrium. Below a threshold frequency, the infection tends to die out, and above it the infection spreads to near fixation. In a simple model, the threshold depends on the balance between the fitness cost of infection and the failure rate of maternal transmission on one side, and the strength of incompatibility on the other:

where s_f is the reduction in fecundity that is caused by infection, μ is the proportion of the offspring of infected mothers that are uninfected, and s_h is the proportion of embryos killed in incompatible crosses. For example, if s_f = 0.1, μ = 0.05, and s_h = 0.9, then p_threshold ≈ 0.15 / 0.9 ≈ 0.17. For the strains that are used in mosquito control, estimates of the threshold are typically in the region of 20 to 35 percent, and they depend on the strain and the conditions.

This behavior has two practical consequences. An infection must be introduced above the threshold to establish, which requires sustained releases and not a single one. Once established, it is confined, because a few stray migrants that arrive in a distant population fall below the local threshold and die out.

In some hosts, Wolbachia has become beneficial or essential and not parasitic.

• Filarial nematodes. Most of the worms that cause human disease, including Onchocerca volvulus (river blindness), Wuchereria bancrofti, and Brugia malayi (lymphatic filariasis), carry Wolbachia in every individual and cannot complete their life cycle without it. The bacteria appear to supply nucleotides, riboflavin, and heme, which these worms cannot make. A notable exception is Loa loa, the African eye worm, which lacks the bacteria.
• Bedbugs. The strain _w_Cle in Cimex lectularius supplies biotin and riboflavin, which are scarce in a diet of blood. Bedbugs that are cleared of the bacteria grow poorly and reproduce less, and are restored by vitamin supplementation.
• Fruit flies and others. Some strains improve host fecundity, protect against viruses, or supply iron under nutritional stress. In at least one laboratory population of Drosophila, a strain has shifted from costly to beneficial over a few decades. In the parasitoid wasp Asobara tabida, Wolbachia is required for oogenesis.

Anti-_Wolbachia_ therapy for filarial disease

Because the filarial worms depend on their bacteria, Wolbachia is a drug target. Treating patients with doxycycline for several weeks kills the bacteria, after which the worms become sterile and eventually die. This is slower than conventional antiparasitic drugs, but it produces lasting sterilization, and it avoids the severe reactions that can follow rapid killing of worms. The long course, and the fact that doxycycline cannot be given to young children or in pregnancy, have limited its use, and shorter-course alternatives, such as the combination of rifampicin with other drugs and new compounds, are in development. The approach is also attractive where Loa loa is co-endemic, since conventional treatment in that case risks severe complications, and Loa loa is unaffected by anti-_Wolbachia_ drugs.

In 2008, it was found that Drosophila that carried certain Wolbachia strains survived infection with RNA viruses that killed uninfected flies. The effect was soon shown to extend to mosquitoes that were artificially infected with Wolbachia, which transmitted dengue virus and related viruses far less efficiently. Because Aedes aegypti does not naturally carry Wolbachia, its effect was first studied in artificially infected lines.

The mechanism is not fully resolved, and several processes probably contribute:

• Competition for lipids. Both the bacterium and the viruses depend on host cholesterol and on membranes derived from the endoplasmic reticulum. Wolbachia appears to sequester cholesterol and remodel these membranes, leaving less available for viral replication.
• Priming of host defenses. Infected mosquitoes show raised baseline activity of immune pathways, including the Toll pathway, and of reactive oxygen species.
• Interference with host RNA and protein handling, including effects on the cellular machinery that the virus requires.

The strength of blocking varies among bacterial strains, host species, and viruses, and it generally correlates with the density of the bacteria in the host tissues. Denser infections block viruses more effectively, but they also impose greater costs on the mosquito. Blocking has been shown for dengue, Zika, and chikungunya viruses, and it is weaker or absent for some other viruses, and some studies have found enhancement of other pathogens in some host-strain combinations, so the effect is not universal.

Replacing mosquito populations

Aedes aegypti, the main vector of dengue, does not naturally carry Wolbachia. In the 2000s and 2010s, Scott O'Neill and colleagues established stable infections in this species by injecting bacteria into mosquito embryos, using strains taken from Drosophila (_w_Mel) and from Aedes albopictus (_w_AlbB).

Released mosquitoes spread the infection through the local population by cytoplasmic incompatibility, provided that releases are sustained until the frequency passes the threshold. Once established, the infection can persist without further intervention. In Cairns, Australia, where releases of _w_Mel began in 2011, the infection has remained in place for more than a decade.

The approach was tested in a randomized controlled trial in Yogyakarta, Indonesia, reported by Utarini, Indriani, Simmons, and colleagues in 2021, which divided the city into treated and untreated clusters. Symptomatic dengue fell by 77 percent and hospital admissions by 86 percent in treated areas. Deployments have since been made in Brazil, Colombia, Vietnam, Malaysia, and elsewhere, and several have reported large reductions, though these studies were not randomized and are harder to interpret. Because population replacement depends on the spread of the infection, it is a long-lasting measure that needs less repeated effort than methods that require continual releases.

A practical limitation is temperature. The _w_Mel strain loses density in larval containers that heat above the mid-30s °C, which can slow the establishment of the infection in very hot climates. The more heat-tolerant _w_AlbB has been used in such settings, including Malaysia. Other concerns include the possible evolution of the virus or the mosquito, the effect on non-target species, and public acceptance, which has been addressed by community engagement.

Suppressing mosquito populations

The same Wolbachia incompatibility can be used in the opposite way. Releasing only infected males into a population that has no Wolbachia, or one with an incompatible strain, makes the wild females that they mate with produce no viable eggs, without establishing the bacterium in the field. This is the incompatible insect technique (IIT), and it is related to the older sterile insect technique.

It requires continual releases and accurate separation of the sexes, since releasing even a few infected females would establish the infection and end the suppression. Programs therefore combine automated sorting with a low dose of radiation to sterilize any females that slip through, a combination that is called the combined IIT-SIT approach. The approach has reduced mosquito numbers substantially in trials in Singapore, parts of China, and the United States, including California, and it has also been used in efforts to protect native Hawaiian forest birds from mosquito-borne avian malaria.

Other targets

Wolbachia strains have been introduced into Aedes albopictus and are being investigated in Anopheles mosquitoes for malaria control, where natural infections are uncommon and establishing stable ones has been difficult. Interest also exists in agricultural pests.

Because Wolbachia manipulates reproduction, it affects the evolution of its hosts. It can alter patterns of mitochondrial DNA diversity, since mitochondria are inherited with the bacteria, and this can mislead phylogenetic and population studies that depend on mitochondrial markers. It may also contribute to speciation through the reproductive isolation that incompatibility creates, although the importance of this process in nature is debated. Research questions include how the cif genes work, how often virus blocking occurs in natural populations, how long deployed strains remain stable, and whether hosts evolve resistance to incompatibility or to blocking.

How do the bacteriophage WO genes *cifA* and *cifB* cause cytoplasmic incompatibility?

In *Wolbachia*-infected male insects, the prophage WO operon genes *cifA* and *cifB* are co-expressed during spermatogenesis, where the CifB deubiquitylase or nuclease modifies paternal chromatin before the bacteria are stripped from maturing sperm. When this modified sperm fertilizes an uninfected egg lacking maternally deposited CifA protein, paternal chromosomes fail to decondense and segregate synchronously during the first mitotic division, arresting embryogenesis; in *Wolbachia*-infected eggs, maternal CifA binds CifB or reverses the chromatin modification, rescuing viability.

Why is there an unstable threshold frequency for *Wolbachia* invasion in mosquito populations?

Because transinfected *Wolbachia* strains impose a fecundity and larval survival cost (*s*_f) on infected females and exhibit imperfect maternal transmission (*μ*), rare *Wolbachia*-infected females experience net negative selection when most mates are uninfected. Only when the local infection frequency exceeds the threshold *p*_threshold ≈ (*s*_f + *μ*) / *s*_h does the reproductive advantage of cytoplasmic incompatibility (*s*_h)—which sterilizes uninfected females mating with infected males—outweigh the fitness cost, driving self-sustaining spread to near-fixation.

Why does doxycycline sterilize and kill filarial worms in onchocerciasis and lymphatic filariasis?

Filarial nematodes of the species *Onchocerca volvulus*, *Wuchereria bancrofti*, and *Brugia malayi* harbor obligate mutualistic *Wolbachia* (supergroups C and D) inside their hypodermal cords and female germline, relying on the endosymbiont for de novo synthesis of heme, riboflavin, and purine/pyrimidine nucleotides. A 4- to 6-week course of the tetracycline antibiotic doxycycline depletes *Wolbachia* by inhibiting bacterial 30S ribosomal translation, halting embryogenesis in female worms and inducing gradual macrofilaricidal apoptosis over 12 to 18 months without triggering acute inflammatory reactions.

• Vector control
• Cytoplasmic incompatibility
• Endosymbiont
• Aedes aegypti
• Lymphatic filariasis
• Sterile insect technique