Bitten! Why some people are just more attractive… to mosquitoes


 

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Have you ever wondered why you seem to attract every mosquito within a mile radius while the person standing next to you appears completely immune? Or maybe you’re the lucky one that they tend to ignore in their quest for a tastier meal. Either way, there are a number of factors, from your skin microbiome to what you drank with dinner, that can affect your relative attractiveness to mosquitoes.

There are about 2,700 species of mosquitos in the world, but not all of them bite humans. Most prefer birds or other mammals to us. Mosquitoes get their daily nutrition from sipping flower nectar, but when it comes time for the female to reproduce, she needs the protein of a blood meal to develop her eggs. Male mosquitoes don’t drink blood at all.

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                                     A mosquito sips nectar from a flower

 

When a female mosquito lands on your skin, she inserts two tiny tubes. The first one injects an anti-clotting agent, while the second draws out the blood that pools as a result. Most of the time, the bitten human is unharmed, except for the itch caused by our body’s histamine response. But mosquitoes are also vectors for several human diseases, such as Yellow Fever, West Nile Virus, and Malaria. Because of this, we would expect that over time some pretty complex evolutionary interactions have built up as the human, the mosquito, and the parasite or virus all try to come out on top and survive.

And they have…

One major draw to a hungry, human-biting female mosquito is blood type. About 85% of all people are ‘secretors’. They secrete chemical signals through their skin that allow mosquitoes to identify their blood type, and Type O is preferred over all others. Having Type O blood and being a secretor makes you 83% more likely to be bitten. People with Type A are least likely to be bitten, but are also the most likely to suffer severe symptoms or die from untreated malaria. Those with Type O blood are likely to have less serious cases of the disease, due to physiological differences in their blood.

So why might mosquitoes flock to Type O? Humans, mosquitoes and malaria all co-evolved in Africa beginning around 200,000 years ago, so there is a long history between the three of us. If people with Type O blood were always better able to survive long enough to pass the malarial parasite (Plasmodium falciparum) on through another mosquito bite, then they were the ones the parasite would have favored. We already know that Plasmodium can affect the behavior of its mosquito host, because infected mosquitoes have been shown to be more strongly drawn to human odors.

And speaking of human odors…

Body odor is another strong draw for mosquitoes. Human sweat doesn’t smell until the bacteria on our skin begin to break it down, and different bacteria produce different odors, giving us each a unique scent. Smelly feet are especially attractive to mosquitoes, and they will land on them over any other body part. Oddly, they are also attracted to Limburger cheese. Or maybe not so oddly, since Limburger is made by one of the same bacteria that causes foot odor.

Even though we all have about the same total number of bacteria on our skin, some people have a wider variety of species, while others are dominated by just a few types. Studies have shown that people with only a few types of bacteria are more likely to be bitten, even when the total number of bacterial cells is the same. Having a wide variety of bacteria seems to protect us, and since you acquire you skin flora from your mother at birth, they tend to be passed down through families (or at least they were until the age of antibiotics and disinfectants).

Interestingly, people with ancestry from malarial areas of the world are less attractive to mosquitoes than those descended from populations that haven’t had exposure to malaria, despite their blood type. This suggests some level of co-evolution between humans and their skin flora to avoid a deadly disease.

And a few other attractive things…

-Mosquitoes are drawn to the carbon dioxide we exhale. If you are a larger person or have been exercising, you are more likely to be bitten.

-Exercise causes lactic acid to be secreted through your skin, also a draw for mosquitoes.

-Mosquitoes have also been shown to prefer the scent of a beer drinker’s skin.

-One study showed that mosquitoes are 500 times more likely to bite when the moon is full.

 

So… if you are a person with type O blood and low microbial diversity on your skin that has just gone for a run, taken off your shoes while sipping a beer and noticing that the moon is full… prepare to be bitten.

 

 

Smallegenge, R.C. et.al. 2013. Malaria infected mosquitoes express enhanced attraction to human odor. PLoS ONE 8(5)

Verhulst, N.O. et.al. 2009. Cultured skin microbiota attracts malaria mosquitoes. Malaria Journal 8:302

Verhulst, N.O. et.al. 2011. Composition of human skin microbiota affects attractiveness to malaria mosquitoes. PLoS ONE 6(12)

 

Setting up a new colony of honey bees…


 

 

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A new colony of honey bees arrives at the farm…

 

 

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The queen is in a small wooden box…

 

 

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The queen goes into the hive first.  There is a plug in the box made out of sugar ‘candy’ that the worker bees will chew through to let her exit…

 

 

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Now the rest of the bees are shaken out of the box they arrived in, and into their new home…

 

 

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The bees are fed sugar water until the flowers finally bloom in New England…

 

 

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Home sweet home…

The Buzz on Honey


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Honey bees (Apis mellifera) have been around for about 130 million years, and although the human species isn’t nearly as ancient, we have been taking advantage of the bees’ hard work for at least as long as our own recorded history. For thousands of years and across hundreds of cultures, honey has been used as both food and medicine.

Honey bees make honey as a source of sustenance to keep the colony alive through the winter. When a honey bee collects nectar from a flower, it stores it in a special stomach (they have two), where it is treated with enzymes that start to break the nectar down into simple sugars (glucose and fructose). When the bee returns to the hive, it passes off its load of nectar to the worker bees there, who take it into their mouths and add more enzymes and other compounds before storing it in a honeycomb cell. Once in the cell, the bees fan their wings over it until it dehydrates and thickens into the substance we know as honey. The finished honey is more than just sugar; it contains more than 180 different substances, including antioxidants, enzymes, amino acids, vitamins, minerals, and antibacterial compounds like methylglyoxal.

But that’s not all…

Honey has such strong antimicrobial properties that it has been shown to be effective against even antibiotic resistant super germs, and biofilms, which are otherwise hard to treat. A biofilm is a ‘layer’ of bacteria that have come together and pooled their defensive resources, often making them nearly impervious to antibiotics.

The bees depend on the long-term storage of their honey for survival, so contamination of their food source with bacteria or fungus would be devastating. To keep this from happening, the bees add a protein from their own immune systems, called defensin-1, which works in synergy with other antimicrobial properties of honey.

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Honey makes an ideal antimicrobial wound dressing, because under certain conditions, it slowly releases small amounts of hydrogen peroxide. Honey contains glucose and an enzyme called glucose oxidase that can break down the glucose into hydrogen peroxide. Honey itself does not form hydrogen peroxide, because the enzyme needs a pH between 5.5 and 8.0 (honey is around 3 or 4), and a certain amount of sodium to become active. But when honey comes in contact with a human wound, the conditions are just right. In a study of wound patients who had wounds that failed to heal with conventional treatment, application of raw, unfiltered honey resulted in improvement in all but one case, and the infected wounds became completely sterile within one week of application. Honey also has strong anti-inflammatory properties, which aids healing and reduces scarring.

Is all honey the same?

The antimicrobial potency of honey can vary widely, not only due to the nectar sources it was made from, but the way it was processed.

Most of the honey you buy in the grocery store has been filtered, to remove debris, but also to remove small particles like pollen or wax that can speed crystallization. Once the pollen has been removed, it is impossible to tell where the honey has come from. Sometimes honey is smuggled illegally into the U.S. market (often from China), and may be from unapproved sources that could contain heavy metals, antibiotics or other chemicals. There is also the very real possibility of filtered honey being watered down with fructose and other sweeteners by unscrupulous producers.

For maximum benefits, look for raw, unfiltered honey from a reputable source. Heated or filtered honey may be missing many of the enzymes and other beneficial substances.

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Feed your flora…

Honey is not only good for you, but it’s also good for your gut bacteria. Besides fructose and glucose, it also contains about 5% fructooligosaccharides, which are nondigestible carbohydrates. They pass through our stomach and small intestine and into the large intestine, where they become food for the good bacteria such as bifido and lactobacilli. In one study of mice, those fed a diet supplemented with honey had a marked increase in their good gut bacteria.

Why no honey for babies?

The bacteria Clostridium botulinum is found throughout our environment, but it can only grow and multiply where there is no air.  When the bacteria are exposed to air, they form spores that wait around for the right conditions.  When we ingest these spores, our immune system and our gut bacteria easily eliminate them.  But the immune systems of children less than a year old are not mature enough to do this, and the spores can begin to grow and multiply in the absence of air in the gut.  Botulinum spores can get into honey when bees encounter them in the environment.

But for the rest of us…

A little honey in our diets and on our cuts and scrapes might go a long way…

 

References:

El-Arab, A.M.E., et.al. 2006. Effect of dietary honey on intestinal microflora and toxicity of mycotoxins in mice. BMC Complement Altern Med. 6(6):

Efam, S.E.E. 1988. Clinical observations on the wound healing properties of honey. British Journal of Surgery 75(7): 679-681

Kwakman, P.H.S. et.al. 2010. How honey kills bacteria. The FASEB Journal 24(7): 2576-2582

Vallianou, N.G. et.al. 2014. Honey and its anti-inflammatory, anti-bacterial, and anti-oxidant properties. General Med. 2(2)

 

 

 

Anti-bacterial soaps: why they do far more harm than good


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There is an entire aisle in the supermarket devoted to cleaning products, many of which contain antibacterial chemicals. The most common, Triclosan, is found in about 75% of antibacterial soaps. Triclosan (2,4,4’ –trichloro-2’-hydroxydiphenyl ether) is a synthetic antibacterial chemical that was developed in the 1970s for use in hospital scrub rooms, but it has since made its way into hundreds of the products we use every day, all without formal FDA approval or safety studies.  Not only is Triclosan a common ingredient in hand soap, it’s also in children’s toys, toothpaste, deodorant, cosmetics, bedding, clothing, cutting boards, and much more, often marketed under trade names such as Microban®.

It doesn’t stay in those products…

A Center for Disease Control study in 2008 found that 75% of people have Triclosan in their urine. The chemical has also been found in breast milk, amniotic fluid, nasal secretions, and blood plasma, so it is obviously absorbed quite easily into our bodies.

Why is this bad?

Studies in animals and in cultured human cells have shown that Triclosan reduces heart and muscle function by as much as 25%. And this is after just 20 minutes, at doses equal to normal daily exposure. While a healthy person might not notice any difference, this could be a real problem for someone with already impaired cardiac function.

Triclosan has also been shown to alter levels of hormones, including estrogen and testosterone, and can have negative effects on thyroid function. In children, exposure to the chemical increases incidence of allergies. It isn’t yet known if this is a direct effect, or if Triclosan simply kills off too many of a child’s normal, protective bacteria, altering their developing immune function.

Killing off your ‘friendly’ bacteria is never a good idea, because it leaves that formerly occupied space wide open for infection, or to be covered by a bacterium that is resistant to antibacterials. Remember how Triclosan is absorbed into your bloodstream and later turns up in nasal secretions? It turns out that this enables Staph. aureus to bind to certain proteins in the nasal cavity and colonize it. Three in ten people carry Staph naturally in small, harmless amounts, but when it’s given an opportunity to take over, it will, and 85% of Staph infections are caused by our own bacteria

What about all of that Triclosan going down the drain?

Triclosan is one of the most frequently found chemicals in our rivers and streams, and it doesn’t break down easily, so it tends to build up in the environment (and, by the way, in our own bodies). When it reacts with chlorinated water, commonly found in homes in cities and towns with a central water supply, it can form chloroform, a potentially carcinogenic compound.

It does kill bacteria, as promised by those ads…

But it doesn’t work any better than plain soap and water would, even the FDA says so.  And this completely unnecessary chemical is adding to the growing problem of antibiotic resistance.  Triclosan works by blocking an enzyme that bacteria need to make a certain fatty acid that is part of their cell membrane.  The mechanisms bacteria develop to get around this problem and become resistant to Triclosan are often the same mechanisms they use to develop resistance to antibiotics.  Studies have shown that bacteria which have developed resistance to Triclosan are also now resistant to important antibiotics, including erythromycin, ciprofloxacin, ampicillin and gentamicin. All without ever coming in contact with these drugs.

Ironically, Triclosan has no effect at all on viruses, which cause the majority of the illness people are trying to avoid in the first place, such as colds, flu, and the norovirus.

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What about alcohol-based hand sanitizers?

They are safe and relatively effective, as long as the alcohol content is at least 60%. Less than that, and they are too dilute to kill bacteria, with 70% being the most effective concentration. Alcohol kills bacteria and viruses physically, not chemically like antibacterials and antibiotics, so they can’t develop resistance to it. Alcohol disrupts lipids (fats) in the bacterial membrane (its ‘skin’), causing it to begin to fall apart. The alcohol can then enter the cell and denature the proteins there, killing the bacterium. If the percentage of alcohol to water is too high (greater than 75% or so) it evaporates before it can get inside the bacterial cell, and doesn’t do any good. Keeping this in mind, you need to use enough alcohol-based hand sanitizer so that it doesn’t evaporate too quickly from your skin.  Your hands should feel wet for at least 15 seconds, or longer.

And what works best of all for germ-free hands?

That’s right, plain old soap and water. Soap works in two ways. First, it loosens dirt and germs from our skin and washes them away. Second, it has the same kind of physical killing power as alcohol. Remember those fats that hold bacterial cell membranes together? Just think about how well soap breaks up the grease on your dishes. It breaks apart bacteria in much the same way. To eliminate the majority of germs from your hands, you should wash them for at least 24 seconds

Why doesn’t soap and alcohol hurt our own cells?

Our cells have the same kind of lipid-based membrane as bacterial cells do, but we are much better protected. The outer layers of our skin have high amounts of a protein called keratin that forms a tough barrier to protect them from physical assault. As the keratin builds up and gets too thick, the outer cells die, which is why our skin is constantly shed and replaced. When soap gets into a cut, our nasal passages or our eyes, which are not protected by keratin, it stings. This is because the soap is damaging our cells just like it damages bacteria. Your body protects itself from this damage by increasing fluids to wash the irritant away, which is why your eyes and nose water when you get soap in them.

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References:

Allmyr, M. et.al. 2006. Triclosan in plasma and milk from Swedish nursing mothers and their exposure via personal care products. Science of the Total Environment 372(1): 87-93

Calafat, A.M. et.al.  2008.  Urinary concentrations of Triclosan in the U.S. population: 2003-2004. Environ. Health Perspect. 116(3): 303-307

Cherednichenko, G., et.al. 2012. Triclosan impairs excitation-contraction coupling and Ca2+ dynamics in striated muscle. PNAS 109(35)

Rule, K.L, Ebbett, V.R., and Vikesland, P.J. 2005. Formation of chloroform and chlorinated organics by free-chlorine-mediated oxidation of Triclosan. Environ. Sci. Technol. 39(9): 3176-3185

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Chickens Eggs and Salmonella


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‘Bernice’, our farm’s favorite chicken

 

Salmonella is a type of bacteria that was discovered in 1887 by an American veterinarian named Daniel Elmer Salmon (hence the name), and there are over 2,000 strains that we know of. Salmonella is actually quite common in the environment and even in our own gut, but luckily only some of those thousands of strains are harmful to humans. Many of those harmful strains happen to be carried by birds and other animals, including chickens, as a small part of their normal gut flora. All chickens can have varying amounts of Salmonella in their bodies, no matter how clean their surroundings are kept.

So how does Salmonella get into eggs?

It usually doesn’t. Nature has worked very hard to ensure that the inside of an egg is protected from Salmonella and other bacteria in the environment. When a hen lays an egg, she also deposits a natural antibacterial coating over the surface, often called the “bloom” or the “cuticle”. After all, eggs are meant to be incubated by the hen for three weeks, and if bacteria get inside during that time, the developing embryo would most likely die. There are lots of tiny pores in the eggshell that allow oxygen to get in, and the antibacterial bloom helps to keep bacteria from entering as well.

Since chickens carry Salmonella within their bodies, occasionally a hen might lay an egg with a small amount of Salmonella inside. This happens very rarely. On average 1 in every 20,000 eggs laid contains Salmonella bacteria, and even then it’s a tiny amount, most often five or less bacteria. It usually takes at least 100 Salmonella bacteria to make someone sick.  Bacteria can sometimes get in through the shell, especially if it’s damaged or cracked.

     Factory farm eggs increase risk

In factory farms, thousands upon thousands of chickens are crowded together, making it very easy for diseases to spread. Baby chicks are raised in isolation from adults and also fed antibiotics, which means they never acquire the natural gut microbes that normally protect adult birds from being colonized by large numbers of potential human pathogens like Salmonella.

In this environment, the bacteria that do colonize poultry tend to be the ones that have become resistant to the antibiotics the hens are fed. Chickens under high stress are also more likely to have large amounts of Salmonella, since stress hormones have been shown to increase the bacteria’s growth rate.

To wash or not to wash…

Washing removes most of the protective bloom from the surface of the shell, but In the U.S., producers are required to wash their eggs, at a temperature at least twenty degrees warmer than the inside of the egg. The logic behind the higher temperature is that cold water could cause the contents of the egg to contract, drawing contaminants inside through the pores. After washing, commercially produced eggs are then rinsed with a chemical sanitizer (which can also enter the egg) and dried, because bacteria cannot go through the pores of an egg without moisture as a vehicle to carry them across.

In Europe, there are very different thoughts on egg safety. European laws prevent producers from washing eggs at all, so that the natural antibacterial “bloom” remains intact. In European supermarkets, eggs are not refrigerated, because if they begin to warm up as you bring them home, condensation can form on the surface, and since bacteria needs a moist surface in order to enter an egg, this increases that chance of bacteria getting inside.

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freshly laid eggs from our own flock…

And buying eggs from your local farmer?

In contrast to large factory farms, happy, free-roaming hens are less likely to be stressed and more likely to have a normal, diverse gut flora that are in competition with each other and preventing a prevalence of pathogens like Salmonella. In uncrowded conditions, there is less likely to be a large build-up of bacteria in the environment, and in the absence of antibiotics, diverse communities of bacteria vie with each other for resources, not allowing any one type to become dominant.

So are eggs from small farms safer? I know which ones I’d rather eat…

 

 

 

 

 

 

 

H. pylori: old friend, new enemy?


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Up until the 1980s, doctors and scientists believed that gastritis and stomach ulcers were caused by peptic acid eroding the lining of the stomach. Then, two scientists from Australia, Dr. Barry J. Marshall and Dr. J. Robin Warren, made a discovery that changed everything: most people who suffered from ulcers also had a bacterial infection that was causing inflammation of the stomach lining. The newly discovered bacterium was named Helicobacter pylori, and doctors soon found that treatment with antibiotics could effectively cure most ulcers.  Good news for ulcer patients.

In 2005, Marshall and Warren were awarded the Nobel Prize for Physiology or Medicine for discovering H. pylori and its role in gastric disease, which led to wide-spread news and media coverage. People everywhere were hearing about this ‘bad stomach bacteria that you didn’t want to have’, but a few years have passed, and we are beginning to understand that it’s just not that simple.  You might want to have it, or you might not… it depends…

We now know that humans (along with many species of animals and birds) have been colonized by H. pylori for at least 60,000 years, and probably longer. We know this because geneticists can calculate average mutation rates of the bacterium’s DNA as it travelled along in the first Homo sapien exodus from Africa.

We seem to have lived together quite peacefully with H. pylori until very recently in human history. Not long ago, just about every human on the planet was colonized with H. pylori. It was (and still is) part of our normal microbial flora, but today only about half of all people are still carrying it.

Ironically, infection with H. pylori has been linked to gastritis and peptic ulcers, both of which have been on the rise in the last hundred years or so, just when the incidence of H. pylori infection is rapidly declining.  And, peptic ulcers were very uncommon before the 20th century, when just about everyone was infected with H. pylori.

Also practically unheard of before modern times was gastroesophageal reflux disease (GERD), a disease that has been increasing steadily, and is most common in people who aren’t colonized by H. pylori at all. GERD is not common in people who have H. pylori. So, here we have a bacteria that we apparently lived peacefully with for much, if not all, of our time as Homo sapiens, not even knowing it was there, and now suddenly we can’t live with it, can’t live without it…

What went wrong?

As it turns out, being colonized with H. pylori causes your stomach to produce less acid, and since humans have been colonized with the bacterium for so long, the ‘less acid’ state has become normal for us. Now, remove the H. pylori from the equation, and what happens? That’s right, a lot more acid is produced, and the incidence of GERD goes up as well.

But then why the ulcers?

Most people who are colonized with H. Pylori never get ulcers, but those that do have an abnormally low Treg response in their gastric system. ‘Treg’ is short for ‘regulatory T cells’, and their job is to keep the immune system in check, for example turning off the inflammatory response after fighting off a disease, or preventing the immune system from attacking things that aren’t really dangerous to us (and causing allergic reactions).

We have learned that much of our Treg response depends on our immune system learning early on in childhood what is friend and what is foe, and exposure to a large variety of bacteria early in life is important for all that to be sorted out. Yet with the advent of antibiotics, disinfectants, and improved sanitation, we are exposed to far less bacteria, both ‘good’ and ‘bad’, now than ever before in our history. Studies have already shown that children raised on farms where they are exposed to a wide variety of bacteria have far less incidence of asthma and other autoimmune diseases.

When we are not exposed to H. pylori as a child, our immune system does not recognize it as part of our normal gut flora when we acquire the bacteria as an adult. This means the Treg response doesn’t effectively control inflammation in the lining of the stomach, and ulcers can result. The excessive use of antibiotics may also have selected more virulent strains, which outcompete the more harmless ones..

And cancer?

Yes, prolonged infection with H. pylori is linked to stomach cancer, or at least it has been in the recent past, and here’s where things get even more complex. It seems that you need pretty close contact in order to pass H. pylori from person to person, and therefore in the history of our co-evolution, families, tribes, and other close-knit groups tend to share the same strain of the bacteria. Over time, people and their local strain of H. pylori adapt to one another’s small differences.

But now people have started moving around faster and farther than ever before, and are constantly encountering new strains of H. pylori. If you’re already colonized by your own familiar strain, that’s not really a problem. But if you acquire H. pylori for the first time from a ‘stranger’, it’s more likely not to agree with you and cause inflammation and other changes that might eventually lead to cancer.

A fascinating study of two populations in Colombia illustrates how closely H. pylori has evolved with its human hosts. One of these populations lives in the mountains, and is of Amerindian descent. The other population, living near the coast, is of largely African descent. Both populations have the same rate of colonization with H. pylori, yet gastric cancer rates are much higher among the Amerindian population.

Why?

When scientists sequenced the genomes of the H. pylori, they found that the bacterial strains colonizing the coastal population were largely of African descent, as were the people. But the bacteria colonizing the Amerindian population were largely Southern European. This mismatch seems to have led to a greater incidence of cancer.

Unfortunately, the absence of H. pylori also leads to an increase in cancer, because of the association of GERD with esophageal changes that can eventually become malignant.

We still have lots to learn about our physiological relationship with microbes, but now that the microbiome has moved into the spotlight, we are on the verge of a new age of understanding, and perhaps a more targeted and refined approach to medical care.

 

References

Atherton, J.C. and Blaser, M.J. 2009. Coadaptation of Helicobacter pylori and humans: ancient history, modern implications. J Clin Invest. 119(9): 2475-2487

De Sablet, T. et.al. 2011. Phylogenetic origin of Helicobacter pylori is a determinant of gastric cancer risk. Gut 60(9): 1189-1195