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BIOLOGICAL AEROSOL MEASUREMENT IN DAIRY PLANTS

bul whcn combined wilh other environmen(a) stresses such as dehydralion, hydrat ion, inadialion, oxidation from oxy-gen or ozone, and effects of various pollutanis. ihc rcsuli is oftcn leihal. Stersky ei at. (68) measured ihe inaciivation of Salmonella newbrunswick aerosolized under various condi-tions. Disiilicd waicr disseminntion in ihc firsi 20 min rcsulicd in D-values ranging from 41 min ai 2I°C and 30% relaiive humidily (RH) lo 206 min ai I0"C and 90% RH. Skim milk disseminalion ai 10®C resulicd in D-valucs ranging from 245 min to 404 min ai 90% and 30% RH and at 21°C from 164 to 470 min. Aerosol izat ion is strcssful for most vegeiative cells so additional siresses from colleclion proccdurcs and growth media musi bc minimized. Aerosoł-ized organisms havc bccn subjecied to mechanical or physio-logical damage which reduces recovery on selectivc media. Stersky and Hcdrick (66) tested the growth inhibition of various combinations of selcctive media on airbome bacre-ria. The ratio of Escherichia coli colonies on violet red bile (VRB), dcsoxycholate (DES), MacConkey (MAC) agar with overlay. standard piąte count agar (SPC) overlay on VRB. and SPC overlay on DES to ihc colonies on SPC was less than 4%. Eosin methylene blue agar showed cxcellcni rccovery (122%) comparcd to SPC. Tergitol and endo agar recovcrcd 23 and 40% respectively. Recovcry of Pseudo-monas spp. on modified selectivc media was greater than that of coliforms. Recovcry rates of airbome Salmonella newbrunswick ranged from < I % on Salmonella-Shigella agar to 118% on MAC/SPC. Rccovery rates werc somewhai improved by impinging onto SPC agar followed by over-laying with the selective media. Rccommended non-selec-tive media for ovcrall microbial rccovery inelude trypticase soy agar, brain-heart infusion agar and Meuller-Hinton agar. Th esc media may also be fortified with blood to neutralize anti-microbial compounds that may be carried into the sampler (44).

Collection fluids are used for some types of aerosol samplers (liquid impingers). The selection of a liquid collection medium is dependent upon the particular organism bcing isolated. In quantitative siudies a medium must be employed which will minimizc boih multiplication and dcath of the organism. The common collection media inelude buffered gelatin (71), phosphatc buffer (48.75), 2% peptone water (22). nutrient broth (25) and gelalin-milk broth (46).

Factors involved in e.xperimental techniques

Research on biological aerosols ofien includes gen-cration, storing and colleclion of aerosols. Some factors in addition to sublcthal injury which will influence experi-mental results are as follows: Sirain of microorganism. Vegetaiivc cells are morc susceptiblc than spores to aerosol Stress. Thcrc may be substantial strain variation for any given species. Growili condilions. The growth medium and growth phase influence susccpiibility of the microorganism to aerosol stress. Aerosol generulion. The degree of shear .stress influences viability, espccially for vegetative cells. If high viability aerosols are to be generated ariificially. a Iow shear force aerosol generator such as spinning top. vibrat-ing needle. or Bcrglund-Liu yibrating orifice aerosol generator should be chosen (20). Two fluid (air-liquid) atomizera (e.g. Collision atomia<at>f/9)) werc ftrst choice dis-persera in the past, but they impose high shear stress. Aerosols bchavedifferently when generated by wet and dry methods. Aerosol particie sne. Particie sizc continuously changes during aerosol storage and collection. Sizc may dccrcasc through evaporation and collision, and may in-ercase through agglomcration and absorption. Spray fluids. Whcn the growth medium and the spray fluids are diffcrcnl, the spray fluid may affect viability. Stersky et al. (68) found that Salmonella which werc aerosol ized from skim milk had greater D-values than those aerosolized from distilled water. Aerosolization of coliforms with skim milk as op-poscd to distilled water resulicd in growth of morc colonies on selective media (66). Spenl culture fluids. di- and tri-saccharidcs and the polyhydric alcohols, sorbitol and inositol provide the best protcction from aerosol generated forccs (20). Aerosol storage. In order to study the fatc of microbes in aerosol, the aerosol should be stored for an extended period of limę under specific condilions such as known relative humidity. irradiation, etc. Special lechniqucs or apparatus such as the vertical wind tunnel (24), microtliread apparatus (51), ot rotating drum (30) arc rcquired. Aerosol collection methods. Colleclion methods greatly influence the recovery of viable particles. A discussion of the prin-ciples applications. advantages and limitations of these methods follows.

AEROSOL SAMPL1NC METHODS

Methods for sampling airbome microorganisms are basically the same as the methods used to sample dust and other airbome particulates. Existing samplers have been modified for the recovery of living biological agents so that the viability of the microorganism is prcservcd wiihout permitting growth. Dimmick and Akera (23) State “Ideał ly, an aerosol sampler for microbiologica! assay should be capable of counting the total number of living airbome particles in a unit volume of air. as well as determining the number of viable units per particie and the size of the particles containing such units. Howcvcr. this presupposes that 100% of the airbome cells. Iiving or dead, can be physically separated from air without killing them during or after sampling.’* Such a sampler has not yet bccn designed (44).

Most of positive samplers (impingers, im pac lora, filiera. etc.) necd vacuum for sampling air. The vacuum pump cxhaust must bc isolated from the area bcing samplcd, as i( may causc crroncous results. Sevcn types of commcrciully available aerosol samplers are listcd in Tablc 1.

Sedimentation melhods

The exposure agar piąte and microscopic slide expo-surc methods rcly on the force of gravity (if the microorganism containing particles are greater than 10 pm) and air currents (all sizes by random chance) to deposit par-łiclcs on a non-selective or selcctive agar surface. Results

JOURNAL Oh hOOI) PROTHCllON. VOl. 52. JULY 1%'J