Could people survive without E coli
The survival of bacteria in the airborne state
How long bacteria can survive in the air and thus potentially transmit infectious diseases is largely determined by their tenacity. This depends on the type of bacteria, but is also influenced by many other factors, such as temperature, humidity, UV radiation or the so-called "open air factor" (caused by biocidal ozone-olefin reaction products). Examinations to date only give a limited representation of the actual conditions in the outside air, since mostly only the influence of individual environmental factors was examined and no real outside air conditions were present in the test systems used. More research is needed here. In particular, in addition to tenacity, the infectivity of bacteria should also be taken into account when assessing the potential for bacterial infectious diseases to spread through the air.
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Airborne bacteria are found in different concentrations in the entire atmosphere of our planet, in caves deep under the earth's surface  up to heights of 70 km . The air is not part of the natural habitat of the bacteria. They get there by chance, naturally through wind  or spray , but also through the excretion of excrement, the shedding of flakes of skin  or through breathing, speaking, coughing and sneezing [6 to 10]. How long bacteria remain viable in the air depends largely on their tenacity (ability to survive). This describes the ability of a microorganism, even under non-optimal conditions, e.g. B. to survive outside of its usual habitat . Most of the bacterial species found in the air are of little medical relevance or are in such low concentrations that they do not pose an immediate health risk. However, in certain work areas, e.g. B. in waste recycling or animal husbandry companies, the number of airborne bacteria is usually greatly increased and more pathogens are detected [12; 13]. Pathogens from the systems can be emitted into the environment via the exhaust air. How far the bacteria are transported and whether and how long they survive the airborne state is largely unclear. For example stayed Mycoplasma hyopneumoniae infectious in an experiment over a distance of 4.7 km . Legionella can also be spread by air over several kilometers . In extreme cases, microorganisms were transported from the Sahara to European mountain lakes by sandstorms and high-altitude winds, where they were detected at least in terms of molecular biology .
In order to be able to estimate and, if necessary, avert potential dangers and risks for residents in the vicinity of bacteria-emitting systems, the spread of certain key organisms (see ) in the environment is usually simulated using computer models . The parameterization of such models currently follows a conservative approach. It is assumed that the bacteria survive the airborne state 100% regardless of its duration. This does not correspond to the current state of knowledge. In order to substantiate the need for research, this article provides an overview of the literature on the most important factors that significantly influence the survival of airborne bacteria in the outside air - and thus also the risk of disease transmission through the air. The aim in the future is to estimate how long and under what conditions certain bacteria in the outside air can survive and thus remain as potential pathogens. This information can also be used to complete input parameters for dispersion models.
2 Influence of various factors on the tenacity of bacteria in the air
The tenacity of different bacterial species and even strains and isolates within the same species can vary widely [19 to 21]. The tenacities of vegetative forms and permanent forms (e.g. spores) within a species also differ. Thus, bacterial spores of the genus Bacillus in contrast to their vegetative forms, they can even withstand extreme space conditions over a long period of time . In the literature, contradicting information on the tenacity of individual species is also often given . In general, however, gram-positive bacteria seem to be more resistant than gram-negative bacteria . In addition, many other factors influence the tenacity of bacteria , such as B. the cultivation and growth conditions and the method of production of the test bioaerosol [21; 24 to 30], method or type of aerosolization [19; 25; 31 to 33], temperature [25 to 40], humidity [20; 26; 31; 32; 37; 38; 41 to 49], global or ultraviolet (UV) radiation [50 to 56], the pollutant gas content (including oxygen) [20; 29; 31; 43; 57] or the collection technique used [19; 47; 58] as well as the subsequent storage and processing of the samples [31; 41]. According to current knowledge, the greatest influence on the viability of bacteria in the air has meteorological factors and the concentrations of certain biocidal substances in the air, such as free radicals, ozone and ozone-olefin reaction products, which are collectively referred to as "Open Air Factor" (OAF ) are designated [59 to 62]. All factors not only influence one another physically, their effects in the bacterial cells also depend on one another. The exact relationships are complicated and the effects on different groups of microorganisms can be different [57; 63].
2.1 Temperature and humidity
In general, the killing rate of airborne bacteria increases with increasing temperatures [34; 35]. Most cultivable bacteria are found in the outside air at temperatures between 8 and 24 ° C . Temperatures below 8 ° C hardly have a negative impact on survivability. An increase in temperature above 24 ° C, however, leads to a rapid decrease in the bacterial concentration, especially at temperatures between 30 and 40 ° C [39; 49; 63]. Survival at different temperatures also depends on the relative humidity (RH) [20; 30; 38; 39; 42; 47 to 49; 65]. For example, the median survival rate for Escherichia coli at 50% RH and a temperature of 15 ° C approx. 14 minutes, at 30 ° C only three minutes. At 85% RH it is 83 min at 15 ° C and 14 min at 30 ° C . The influence of air humidity on bacteria in the airborne state is more complex than the influence of temperature and is also very dependent on the species . It is therefore difficult to make general statements [66; 67]. For example survived Mycoplasma best at <25% RH and> 80% RH , for Chlamydia pneumoniae on the other hand, a relative humidity of> 95% is favorable. In general, however, very high and very low humidity levels of <20% RH and> 85% RH are considered to reduce viability [20; 49]. A sharp change in humidity - while the bacteria are in the air - also leads to a reduction in viability [68 to 70]. The type of aerosolization thus also has a strong influence on tenacity. If bacteria are released into the air from a suspension, they lose free water and dry up; dry bacterial cells, on the other hand, tend to absorb moisture. This change in water content has been shown to influence tenacity [69; 71; 72]. Become bacteria of the genus Pasteurella When wet, their minimum survival rate is 50% RH. If, on the other hand, they are dispersed dry, it is 75% RH [51; 73].
2.2 UV radiation
Based on the natural spectrum of sunlight, UV radiation has the greatest influence on the tenacity of airborne bacteria. This radiation is divided into UV-A (380 to 315 nm), UV-B (315 to 280 nm) and UV-C (280 to 200 nm) according to its biological effect . UV-C radiation is of no relevance near the earth's surface, as it is already absorbed by the ozone in the upper layers of the atmosphere. Nevertheless, many studies on the effect of UV radiation on the tenacity of airborne bacteria were carried out with UV-C radiation with a wavelength of 253.7 nm, as this can be generated very easily with mercury lamps. When interpreting these results, it should be noted that DNA (DNA: deoxyribonucleic acid, deoxyribonucleic acid) is particularly badly damaged by radiation of this wavelength due to its absorption spectrum. UV radiation with a wavelength of 253.7 nm is therefore used to disinfect water [75; 76] and air was used . The influence of UV-C radiation on airborne microorganisms depends heavily on the water content of the cells during irradiation [50; 52] and this in turn depends on the relative humidity. UV-induced killing and mutation rates increase sharply with the degree of desiccation of a cell . Dehydrated cells are already effectively killed by UV-A and UV-B radiation, whereas moist (fully turgescent) cells are hardly killed . Resistance to the effects of UV-C radiation increases, particularly at a relative humidity of over 80%. B. from Seratia marcescens or Mycobacterium bovis strongly on . Photo reactivation also only takes place in the moist state [50; 52]. In this light-dependent process (approx. 300 to 500 nm wavelength) bacteria can repair damage in the DNA caused by UV radiation with the help of the enzyme photolyase, so that inactivated cells can be reactivated after a certain time [78; 79]. In this context it must also be taken into account that many bacteria can generally change into a “viable but non-culturable” (VBNC) state for an indefinite period of time under stress and are therefore no longer cultivable, but still viable .
In contrast to UV-C radiation, the influence of natural UV radiation on airborne bacteria has hardly been investigated. Here, in a UV-A and UV-B permeable chamber, a significant influence of sunlight on the survival rate of Mycobacterium parafortuitum found at moderate humidity . Ultimately, the UV component of sunlight in the atmosphere is caused by the splitting of oxygen molecules or nitrogen oxides (e.g. NO2) and the subsequent reaction of the fission products with oxygen is responsible for the formation of ozone, which is part of the open-air factor (OAF).
2.3 The open air factor (OAF)
The OAF means that bacteria generally survive more poorly in the outside air than inside, with otherwise similar meteorological conditions [61; 82]. In particular, nucleic acids and coat proteins of microorganisms are severely damaged by the substances that cause OAF. These primary and secondary zonides, which arise from the reaction of olefins and ozone, are extremely unstable in the air and degrade within a few minutes through reactions with particles or surfaces. Therefore, in a closed system, air changes of at least 12 times per hour are necessary to maintain the OAF . In addition, the concentrations of the individual substances are extremely variable and are not associated with a time of day or season . In addition, different olefins and ozone concentrations have different effects on airborne bacteria. For Escherichia coli and Micrococcus albus survival rates of 0 to 100% were found after ten minutes . The humidity also seems to have an influence on the effect of the OAF [61; 62].
2.4 particle size
In most of the studies that examined the influence of temperature, humidity, UV radiation and OAF on airborne bacteria, the microorganisms were largely present as individual cells during the tests. Naturally, however, bacteria predominantly occur in conglomerates . In these, they are to a certain extent protected from the effects of the weather and therefore presumably viable longer under natural conditions than has been determined in laboratory tests. Many authors found a positive correlation between the survival of airborne bacteria and the particle size fraction in which they were detected [25; 54; 61; 65; 83; 84].
2.5 Other influencing factors
In addition to the factors mentioned so far, others also have an influence on the tenacity of airborne bacteria. Cultivated bacteria that were harvested from the stationary growth phase for the experiments survive better in the air than those that were in the log phase at harvest . If the cultures were frozen before aerosolization, this at least affected Mycoplasma pneumoniae the viability of the bacteria is positive . The air humidity also has an influence here. Freeze-dried Pasteurella tularensis, resuspended in distilled water, and bacteria in liquid cultures survive best at high humidity, whereas freeze-dried cells, dry dispersed, survived best at low humidity . The addition of certain substances, such as. B. Di- or trisaccharides, for the suspension to be aerosolized, protect the bacteria in the air from drying out too quickly [26; 66]. The bioaerosol generator used for aerosolization can also have effects on the results of up to a power of ten , as can the collection system used [58; 85; 86]. The bulk liquid is also important; so survived z. B. E. coli in H2O better at low air humidity than in dextran, at high air humidity it is the other way round . Also Pasteurella pestis showed different survival rates at different humidity levels when collected in "Heart Infusion Broth" and Peptone . Ultimately, the virulence of a bacterial strain also seems to influence tenacity or vice versa, as for Legionella pneumophila has been shown . Here the most virulent strain was the one with the highest tenacity.
3 test systems for examining tenacity
In order to be able to assess the results of the presented studies on the tenacity of airborne bacteria, it is important to look at the test systems and test conditions. The table provides an overview of the systems used, the factors examined and the test organisms.
Overview of the bioaerosol chambers used in tenacity studies, the factors examined and test organisms.
Most of the tests were carried out under controlled conditions in static bioaerosol chambers. Were used z. B. Stainless steel cylinder [88; 89] or boxes made of plastic  or aluminum  in which temperature and humidity were regulated. For most of the experiments, the microorganisms were introduced from a suspension into the air inside the chambers with the help of various bioaerosol generators and then collected again from the air using different sampling systems after a defined period of time. The detection of the microorganisms from the air was mostly carried out by cultivation on nutrient media and by comparing the colonies grown under different test conditions. Since the particles sediment in static aerosol chambers, fans were sometimes used to mix the air, but they also acted as impactors and also removed particles from the air [49; 90]. Alternatively, rotating closed cylinders were used as static bioaerosol chambers, in which sedimented microorganisms were repeatedly transferred into the air by rotation [29; 39; 44; 45; 47; 48; 57; 59; 65]. Some authors used a dynamic bioaerosol chamber, in which, however, microorganisms are constantly being lost due to the continuous exchange of air . The test channels used [68; 69] allow only very short observation times.
In the classic bioaerosol test chambers, only the influence of individual factors on the survival of airborne microorganisms can usually be examined. So far, the OAF has received the least consideration, as the substances causing the effect degrade within a few minutes . Clauss et al. report on a bioaerosol chamber made of a UV-permeable foil balloon which is continuously filled with fresh outside air and in which the ozone concentration and thus probably also the OAF could be maintained at 75% for 20 minutes . Some authors have attached bacteria to thin spider threads (microthread technique) to investigate their tenacity in the air [28; 61; 62; 81; 83]. However, the cultivation of the spiders and the “harvest” of the threads is very complex and the subsequent evaluation is difficult, since the bacteria have to be washed off the threads again after the experiments. In addition, many cells were lost before and during the experiments. Ultimately, particles adhering to a surface can only be compared to a limited extent with those that are in an airborne state .To date it has not been possible to investigate the tenacity of airborne bacteria under real outdoor air conditions. There is a need for further research here.
Last but not least, in the event of a possible transmission of bacterially caused diseases, not only the tenacity of a pathogen is relevant, but also the infectivity. This describes the ability of a pathogen to infect a host  and is dependent on various virulence and pathogenicity factors which, like tenacity, can be influenced by the airborne state. For example, the infectivity went from Chlamydia pneumoniae due to the airborne condition . It would therefore be of great interest, not least from an epidemiological point of view, to investigate the influence of the airborne state on the infectiousness of bacteria, which has hitherto hardly been considered.
Due to the great differences between the individual species, only a few general statements can be made about the tenacity of bacteria in the airborne state. Examinations carried out so far can only provide information on possible transmission distances. At the moment, they can only be used to a limited extent as input parameters for improving spread forecasts, also because it has not yet been possible to investigate the tenacity of airborne bacteria under real outdoor air conditions. In addition, due to the different test conditions and the large number of influencing parameters, test results are often contradictory. In future investigations into tenacity, uniform sum parameters - e.g. B. staphylococci in native dusts - are examined. In addition to tenacity, infectivity should also be taken into account when assessing the risk of bioaerosols and assessing the potential for spreading bacterial infectious diseases through the air.
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Dr. rer. medic. Annette Christiane Clauß née Springorum, Dr. rer nat. Marcus Clauß, Thünen Institute for Agricultural Technology, Braunschweig.
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