Did you know that there are microbes that live in honeybee larvae? They even cause a disease that can kill off the entire colony! It just so happens that the honeybee larva is a common habitat for "Paenibacillus alvei" (12). The honeybee gut has a pH of around 5.2 (18) and the temperature ranges from 34-37°C (17). Honeybees eat pollen and nectar which contains various proteins, sugars, lipids, vitamins, and minerals (7). "P. alvei" is a Gram-positive, aerobic, mesophile that is motile and free-living (6). In addition to this, it is also sporulating and a heterotroph (6). "P. alvei" are pathogenic and produce alveoylsin toxin which plays a role in the microbe’s ability to produce signs in larvae that causes a major bacterial disease in honeybees called American foulbrood (6).
Higher Order Taxa Bacteria, Firmicutes, Bacilli, Bacillales, Paenibacillaceae, Paenibacillus
A phylogenetic tree was created using 16S rRNA genes(20). The method used to construct this unrooted phylogenetic tree was the algorithm of Fitch and Margoliash (9) as implemented in the programs FITCH and KITCH in the program package (PHYLIP version 3.2) developed by Felsenstein (8). From this, it was found that "P. alvei" were most closely related to "Bacillus macerans", "Bacillus macquariensis", and "Bacillus polymyxa". It was determined that the levels of 16S rRNA gene sequence similarity ranged from 93 to 94% (20).
The habitat of "Paenibacillus alvei" is mainly the gut of honeybee larvae (3). However, it was found that a few cases of human infection were caused by this microbe (21). One of these infections includes a urinary tract infection in chronic kidney disease (16). Other habitats that it has been found in include soil, milk, mosquito larvae, and the wax moth (6). "P. alvei" contribute to the environment by producing alveolysin which is a thiol-activated toxin. This causes it to be pathogenic and allows it to produce signs in honeybee larvae that causes American foulbrood which is a major bacterial disease of honeybees (6). This microbe has been found in clinical samples from humans, however, research on this is still being conducted (6). The most common environment of "Paenibacillus alvei", which is the honeybee larvae gut, has a pH of around 5.2, and the temperature ranges from 34-37 °C (26,17). The microbes present in larval feces grew the best in acidified media meaning that they are acid-tolerant microbes. In regard to the oxygen levels, the gut of honeybee larvae is mainly anoxic (25). Honeybee larvae consume royal jelly and worker jelly which go to their gut. Royal jelly and worker jelly contain nectar, glandular material, and pollen (3).
Significance to the Environment
The biggest significance "Paenibacillus alvei" have is on honeybee larvae. This is because it can cause American foulbrood disease and destroy the colony (12). Since honeybees are crop pollinators having an entire colony destroyed would be detrimental to the pollination of crops and other plants. There hasn’t been much information found on the nutrient cycling of "Paenibacillus alvei". However, one of its closest relatives, "Paenibacillus polymyxa", is also found in soil and shares a similar function as "Paenibacillus alvei" when found in soil. "Paenibacillus alvei" have shown to reduce "Salmonella enterica" Serovar Newport on tomato plants which in turn, promotes its growth (1). "Paenibacillus polymyxa" has also prevented activities against common plant pathogens such as "Phytophthora sojae" (24). Since these two microbes share similar functions by both suppressing plant disease and promoting plant growth, it may be possible that they have similar mechanisms and nutrient cycles. It was found that "Paenibacillus polymyxa" enhanced the growth of potato, maize, cucumber, and tomato plants by utilizing insoluble phosphorus and atmospheric nitrogen. "P. polymyxa" utilizes plant-derived nutrients and benefits the plant through a direct or indirect mechanism. Direct mechanisms include fixing atmospheric nitrogen, enhancing plant tolerance to abiotic stress, solubilizing mineral phosphates, and synthesizing phytohormones. Indirect mechanisms of plant-growth promotion include inhibiting phytopathogens, stabilizing soil aggregates, inducing plant systemic resistance against pathogens, and maintaining soil nutrients and structure (24). From this information, it could be hypothesized that "Paenibacillus alvei" share similar mechanisms to promote plant growth and similar nutrient cycling as "Paenibacillus polymyxa". I was not able to find if any environmental human-induced changes influenced "Paenibacillus alvei" evolution, population numbers, or spread. However, excessive use of chemical fertilizer has caused acidification, soil hardening, enhanced nitrogen deposition, and increased emissions of greenhouse gasses which affect crops. These changes weren’t said to affect Paenibacillus strains, instead, Paenibacillus strains are being used to reduce these changes in order to increase agricultural productivity (13). Nutrient Cycling.
Ecological Lifestyle and Interactions
"Paenibacillus alvei" inside of honeybee larvae is parasitic (6). This causes American foulbrood in honeybee larvae. The microbe infects larvae that are 12-48 hours old through contaminated foods. "P. alvei" cause this disease by producing spores in the intestines of the larvae, which is then germinated, making it active (5). When the microbe colonizes the gut after reproducing, they release enzymes and toxins that feed on the larvae’s tissue. When the lining in the gut is breached, the infection spreads to all the tissues and causes death. The larvae then break down into a glue-like consistency. Symptoms of the disease include foul odor, spotted brood pattern, sunken cappings, off-center holes in cappings, larval scale, caramel color of dead larvae, pupal tongue (5). In addition to the evidence of "Paenibacillus alvei" impacting the lives of honeybees it has also been found to impact the lives of plants, but in a positive way. There has been evidence of "Paenibacillus alvei" interacting with "Salmonella enterica" when it is added to tomato plants. They appear to compete because "Paenibacillus alvei" decrease the population of "Salmonella enterica" preventing it from interfering with tomato plant’s growth (1). "Paenibacillus alvei" also interact with "S. aureus", "L. monocytogenes", and "E. coli" in humans through competition due to its antimicrobial activity against them (2).
Significance to Humans
"Paenibacillus alvei" have influenced human society by improving agricultural productivity by promoting plant growth, evolution, and development. The excessive use of chemical fertilizers causes acidification, soil hardening, enhanced nitrogen deposition, and increased emissions of greenhouse gasses (13). The result of this is a decrease in plant growth and productivity which is not good for the agriculture industry. Luckily, the Paenibacillus species has the ability to solubilize phosphorus, fix atmospheric nitrogen, synthesizes siderophore, and produce antimicrobial substances and plant hormones. This is why they’re included in biofertilizers (12). Biofertilizers are natural fertilizers with living organisms that improve soils with no negative outcome (4). There’s no information on "Paenibacillus alvei" being in any current biofertilizers, however, the closest relative, "Paenibacillus polymyxa" does (12). Although "Paenibacillus alvei" hasn’t been included, it has been proven to reduce "Salmonella enterica" in tomato plants, which in turn promotes growth (1). This means it's possible that adding it to biofertilizers will still be beneficial due to its antimicrobial efficacy against food-borne pathogens and tomato bacterial phytopathogens (1). Given that "Paenibacillus alvei" can be added to crops, and honeybees are crop pollinators, if honeybees take it back to their hive, they can cause American foulbrood and destroy the colony (5). Since they are crop pollinators this will also be detrimental to the agriculture industry. Additionally, "Paenibacillus alvei" can cause infections in humans (16), but it was found to also produce compounds with antimicrobial activity against pathogens in humans such as "S. aureus", "L. monocytogenes", and "E. coli" (2). This means that it could possibly be a line of defense against antibiotic-resistant microbes. It hasn’t been proven but maybe "Paenibacillus alvei" is pathogenic when it causes an infection because it may produce more antibiotics to protect itself.
Cell Structure, Metabolism & Life Cycle
"Paenibacillus alvei" is a motile Gram-positive microbe (7) with a bacillus shape which is a rod shape, and a size of 2-5 μm in length and 0.5-0.8 μm in width. The spores that they form are 1.8-2.2/ 0.8 μm, ellipsoidal, deforming, and central/subterminal. When grown on agar it has a circular expansion, irregular margins, a 1-3mm diameter, and a white-yellowish color. The spores that form grow in long rows next to each other on agar (19). The smell of this microbe has not yet been reported. "Paenibacillus alvei" grow best in 2%NaCl, a little bit in 5%NaCl, and not all in 7% NaCl. It is anaerobic and its optimum growth temperature is 28 °C but the maximum growth temperature is 35-45 °C and the minimum growth temperature is 15-20 °C (11). However, the honey bee gut is around 34-37 °C. The best carbon growth sources for this microbe includes glucose, N-Acetylglucosamine, cellobiose, salicin, ribose, maltose, esculin, and glycerol (7). Honey, which is what honeybee larvae consume, contains glucose, cellobiose, sucrose, and maltose meaning that favorable carbon growth sources are present in the honeybee gut larvae (15). "Paenibacillus alvei" are also capable of acid production from glucose and glycerol, indole production, catalase, starch hydrolysis, gelatin liquefaction, casein decomposition, dihudroxyaceton, oxidase, and hydrolysis of esculin (19). Starch hydrolysis may be what helps "Paenibacillus alvei" utilize honey (15). Spores that are formed by "Paenibacillus alvei" have genes that are antibiotic-resistant which protects it from antibiotics that beekeepers can treat the hive with. They also contribute to the spread of antimicrobial resistance and the persistence of infection (23). "Paenibacillus alvei" have the ability to produce chitin-degrading enzymes, which contributes to its invasive capacity as a pathogen by degrading what makes up the exoskeleton of insects, chitin. It also produces a hyaluronate lyase which degrades the connective tissue of eukaryotes. This also contributes to its invasive capacity as a pathogen (7). These unique abilities influence how the organism interacts with its environment by degrading and breaking down connective tissues and exoskeleton of honeybee larvae making them into a glue-like consistency (6).
Genome Structure, Content, and/or Gene Expression
The genome size of "Paenibacillus alvei" is 6.83-Mb (6,830,000 bp). Its genome is circular, with a 46.5% GC. It has 1 chromosome and 4 plasmids. This information can be found in the Genome Project. The reason why this organism was sequenced was to evaluate its pathogenic capacity of honeybees in this Genome Publication (7). The pathogenic capacity is important to understand because if it infects a honeybee larva it can be detrimental to the colony and honeybees are necessary for pollinating crops for humans as well as making honey. We also care that it’s a pathogen because it can help the growth of crops themselves so there has to be a way for them to benefit the crops without the risk of being transported to a hive. The genome tells us that it has the potential to produce polyketides, which have antibiotic activity against other microbes, and nonribosomal peptides (14). The 5 nonribosomal peptides that "Paenibacillus alvei" produce are pamI, pamJ, pamK, pamL, and pamM. It was found that they act as a putative virulence factor in American foulbrood disease (10). This means that it is important during pathogenesis for assisting in the antagonism of microbial competitors, contribution to virulence and motility (10). These things help them survive by allowing them to thrive as a pathogen and spread. It also tells us that this organism has multiple characteristics that allow it to strive as a successful pathogen. Other characteristics include having genes that code for chitin-degrading enzymes and hyaluronate lyases. An interesting feature about this genome, in addition to the chitin-degrading enzymes and hyaluronate lyases, is that it contains putative genes for toxins such as a mosquitocidal toxin, a binary toxin, and alveolysin (7). These interesting features of this genome are important for this organism’s success because they play a role in degrading its host and without it, it can’t kill. Without the ability to kill its host, it can’t spread as well since the spores won’t be exposed in the dead larvae (7).
An interesting aspect of this organism is that it produces antibiotics. This organism has four novel polypeptides called Paenialvin A, Paenialvin B, Paenialvin C, Paenialvin D. Paenialvin A–D have shown antibiotic activity against "S. aureus", methicillin-resistant "Staphylococcus aureus" (MRSA), "Bacillus subtilis", and "E. coli". Paenialvins are positively charged and are believed to act as cationic peptide antibiotics that disrupt the cell membrane. This detail is interesting because even though it makes things that help protect humans against pathogens, it also causes infections in humans (14). It hasn’t been found yet, however; this could possibly help the organism survive when it causes infections in our body by producing more antibodies to protect itself. It influences other microbes by competing with human pathogens such as "S. aureus", "L. monocytogenes", and "E. coli" (14). This feature can be interesting to humans because it can be helpful for future medical production against pathogens. More research should be done on this organism and others like it to see what other potential benefits they hold for humans. As of now, we know that they produce antibodies useful to humans, but it can also cause infection (16). If it can be figured out how to prevent Paenibacillus alvei from causing infections in humans, we may be able to use it in many more medicines.
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