Testing of pathogenic organisms in food Essay


Every twenty-four hours, people interact with bacteriums. Bacteria ( and other micro-organisms ) control the universe, they help modulate the environment by commanding the cycling of elements such as N and O. Until the Haber procedure was created they supplied humanity with the N with which to turn harvests with. Bacteria besides live indoors worlds, assisting to digest nutrient and minerals which enzymes could non and supplying much needed vitamins. But non all bacteriums is good, some will do us to fall ailment ( stomach ulcers, nutrient toxic condition, and etc ) . Salmonella in specific was one of the most fecund of all nutrient poisoning beings for the 20Thursdaycentury and still remains a job to this twenty-four hours doing nutrient toxic condition, typhoid febrility, and paratyphoid febrility ( Hardy, 2003 ) . As such it is really of import to be able to place bacteriums that can harm worlds. How would one travel to be able place if bacteriums that can harm is present in things such as nutrient? The reply is simple, to analyze bacteriums so the differentiation can be made. This can be done by taking a sample of nutrient for case and thining it so that you can insulate and find what bacterium is in your sample.

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Frequently times to analyze bacteriums in a sample at that place needs to be an enrichment measure to increase the sum of the bacteriums in your sample so that it would be easier to insulate as merely little sums of the being will be found ( for case, merely a little sum Salmonella is needed to do a individual ill ) while other micro-organisms can proliferate easy ( such as Staphylococcus aureus, would necessitate big figure of settlements to do a individual ill ) . After guaranting there are favourable conditions for whatever bacteriums you’re proving for, it becomes a affair of taking some samples of what you found and executing assorted trials to corroborate what has grown and whether or non it is contaminated.

For the intents of this experiment the experimenters applied the theory from above. When insulating and observing for species such as salmonella it is of import to understand the biochemical reactions are happening for it to be noticeable. For the intents of this experiment we will be observing for the presence of Salmonella and Staphylococcus aureus in natural beefburger meat.

In industry, sensing is of critical importance. Not observing Staphylococcus aureus or Salmonella can take to fatal consequences for consumers. Contamination of nutrient can come from a assortment of beginnings whether it be improper hygiene when fixing natural beefburger meat, contact with septic animate beings, or being infected with person who carries it but shows no symptoms. For case in the domestic fowl and egg concern, to prove for Salmonella in big batches of eggs and domestic fowl, they will take little samples to offsite research labs where they will be tested for Salmonella ( Mitchell, 2010 ) . Typically this would necessitate a turnaround clip of at least 5 yearss, which can take to dearly-won callbacks. But with newer promotions trials can be done within a twenty-four hours ( Mitchell, 2010 ) . The method they use is the PCR procedure which involves magnifying and copying the Deoxyribonucleic acid of Salmonella many times to let for easier sensing in other trials ( Mitchell, 2010 ) . Though this has led to many false positive consequences ( Mitchell, 2010 ) . Newer promotions name for the usage of dyes that merely affect dead Salmonella cells ( which can’t injuries worlds ) leting for easier distinguishing of dead and unrecorded cells.

This is really of import because Staphylococcus aureus and Salmonella are infective micro-organisms that produce toxins. Staphylococcus aureus and Salmonella can come in into nutrient merchandise through improper hygiene, natural nutrients, and eggs to advert a few. Staphylococcus aureus enters into nutrient and excretes a heat stable exotoxin, when ingested is what makes a individual ill. While Salmonella, by lasting through the transition of a person’s tummy, attach onto a person’s bowels and get down to retroflex. Salmonella so produces endotoxins after turning in the enteric piece of land which leads to the symptoms of dysentery/salmonellosis or in utmost instances where the bacterium gets into a person’s blood stream the symptom can attest itself as enteric fever febrility ( Foster, 1996 ) .

As a consequence of their somewhat different methods of infection ( Staphylococcus aureus bring forthing a heat stable exotoxin in the nutrient, while Salmonella produces endotoxins in the bowels ) they both require different media in order to be cultured on. For case there is a strong relationship between strains capable of bring forthing the enzyme coagulase and bring forthing the exotoxin. So by executing a coagulase trial it would be simple to find if there was Staphylococcus aureus nowadays. Whereas for Salmonella utilizing selective ( would be conditions that are merely favourable to Salmonellae growing ) and differential ( dyes in agar doing it easier to observe single settlements of Salmonella ) media for turning salmonella samples ( as there wouldn’t be much in a sample without enrichment ) makes it easy to place the presence of single settlements. In add-on for Salmonella because it produces endotoxins ( LPS in the outer membrane ) we can observe for the presence of Lactose, H2S, Gas formation, and the use of Dextrose to find if Salmonella is present.

The intent of this experiment is to be able to place Staphylococci and Salmonella on common nutrient points and reinforce techniques for research lab experiments.

Methods and Materials

Please refer to Department of Biology, 2015, Biology 241 Introduction to the microbic universe lab manual, Experiment 4: Food unwellnesss caused by staphylococcus and salmonellae, pp. 41-45. All methods and stuffs were followed without any divergence except for the followers.

The followers are the exclusions to the protocols described in the manual:

  • For the initial sensing of Salmonella utilizing the MacConkey agar the consequences used were from the TA sample due to non utilizing the Selenite-Cystine civilization as it went losing during the experiment.


Table 1, Consequences from Enumeration and Enrichment of Staphylococcus



Table 1 summarizes findings of experiment with regard to the settlements counted on the Staphylococcus 110 Agar. Where the Numberss shown supra are the organisms/gram of sample found harmonizing to the dilution factor above it

The coagulase trial was positive when applied to a suspected sample of Staphylococcus Aureus bespeaking that Staphylococcus Aureus was present. This was confirmed by the prescense of a heavy house coagulum. Whereas a negative reaction would hold had no coagulum.

MacConkey Agar had growing after being stray and incubated but the growing was pink. ( TA provided sample )

Table 2, Consequences from farther sensing of Salmonella

Lactose/Sucrose usedDextrose usedH2S producedGas Formation

Result — +-

Table 2 summarizes findings of experiment with regard to the indicant of Salmonella being present after taking stray settlements from the MacConkey Agar. Where ‘+’ indicates a positive reaction shown by bulk salmonella of what was stated above. While ‘-‘ indicates a negative reaction shown by bulk salmonella of what was stated supra. The Triple Sugar Iron Agar trial determined if there was Salmonella in the unknown sample


Given these consequences it would look to connote that in this sample there was merely Staphylococcus Aureus and no Salmonella.

During the first portion of the experiment possible Staphylococcus Aureus was isolated and enriched on Staphylococcus 110 agar. It was found that settlements grew on it. This was because Staphylococcus 110 agar is a selective medium for the isolation and distinction of infective staphylococcuss based on salt tolerance, pigmentation, mannitol agitation and gelatin liquefaction ( Smuckler and Appleman, 1964 ) . A selective medium will merely let certain beings to proliferate while stamp downing others. Hence it would do sense that many settlements of Staphylococcus would turn on this agar. So what can be determined is that anything that can turn on Staphylococcus 110 agar would necessitate to be a infective Staphylococci. Furthermore this would connote that in the beefburger sample provided there was Staphylococcus. So while this does corroborate Staphylococci it does non corroborate Staphylococcus Aureus.

To corroborate the presence of Staphylococcus Aureus in the beefburger sample provided a coagulase trial would be performed on the Staphylococci. This was performed by adding beads of civilization into a BHI stock which contained settlements from the sample earlier. It has already been established that there is a strong relationship between strains capable of bring forthing coagulase and bring forthing the exotoxin. Coagulase is an extracellular protein which binds to prothrombin in the host to organize a composite called staphylothrombin ( Dinges et al, 2000 ) . This activity consequences in the transition of factor I to fibrin, which causes the fibrin of blood plasma to coagulate. This is the footing for the placing marker of a successful coagulase test-the heavy, steadfast clot- ( Dinges et al, 2000 ) . Although coagulase is non expected to be deadly, some speculate that the bacteriums can protect itself from host defences with the curdling provided by coagulase ( Dinges et al, 2000 ) . It is suspected that with coagulase, fibrin-coated staphylococcus resist phagocytosis, doing the bacteriums more deadly ( Foster, 1996 ) . Staphylococcus aureus green goodss free coagulase while other Stapylococci do non, hence coagulase is a traditional marker for placing Staphylococcus Aureus in the clinical microbiology research lab ( Foster, 1996 ) .

In the sensing of Salmonella, Salmonella was isolated by puting the beefburger sample in Selenite-Cystine civilization stock. This would insulate for salmonella in the sample. This is because Selenite-Cystine civilization is specifically used for the selective enrichment of Salmonella species ( Hammack et al, 1999 ) . This is because it is a selective media that will merely forbid the growing of Salmonella species ( Hammack et al, 1999 ) . Therefore if there was Salmonella, it can be said that this broth would insulate for it and supply conditions favourable for growing. Potential beginnings of mistake when incubating the beefburger sample in this was that it was premade so at that place might hold been a opportunity it was made improperly or that the incubation temperature was non 37OC.

A sample of what was grown in the Selenite-Cystine stock was so placed in MacConkey Agar and left to incubate at 37OC for 24 hours. To understand how the MacConkey Agar can distinguish for Salmonella it is of import to understand what it is. MacConkey Agar is considered selective media because it inhibits the growing of gm positive bacteriums due to the presence of crystal violets and gall salts, while leting gram negative bacteriums to boom ( McGuckin et al, 1980 ) . MacConkey agar is besides differential media because distinguishes between bacteriums on the footing of colour, the agar contains ingredients that make it differential: impersonal ruddy ( a pH index and milk sugar ( McGuckin et al, 1980 ) . What happens is bacteriums that are lactose fermenters will make an acidic terminal merchandise which would instance the pH index to turn pink due to the pH alteration, so for this agar it’s non the media that alterations color but the settlements that are utilizing the provided milk sugar, while non-lactose fermenting bacterium will stay colourless McGuckin et Al, 1980 ) . Since Salmonella is a non-lactose zymosis, non-coliform bacteriums if it were to turn on MacConkey agar it would hold been colourless unlike the sample we had received from the TA’s. On this sample it was found to be pink growing, connoting that lactose agitation had occurred and in order for growing to happen the bacteriums would necessitate to be gram negative. This would most likely indicate that another bacterium had managed to acquire into the sample to turn polluting the sample. The lone beginnings of mistake on this portion could come from the taint of the MacConkey agar.

To corroborate the findings above settlements from the MacConkey agar were taken and placed in Triple Sugar Iron Agar ( to be referred to as TSI Slant ) to prove for the presence of Salmonella. These trials included: testing for Lactose and/or Sucrose use, Dextrose use, H2S production, and Gas Formation. To better understand what a TSI angle is and what it tests for. A TSI angle is composed of Lactose, Sucrose, and Glucose ( dextroglucose ) -in 10:10:1 concentration respectively- , Iron ( Ferrous sulphate ) , Phenol red, and Peptone ( Lehman, 2005 ) . When utilizing a TSI slant one of the first things it can prove for is if the bacterium uses Lactose and/or Sucrose, this is identified a xanthous angle for a positive reaction and the slant going ruddy for a negative reaction. This occurs because when lactose or sucrose is fermented big sums of acid ( should be pyruvic acid ) and gas are produced ( Lehman, 2005 ) . Due to acid being introduced from the agitation procedure the Phenol red would turn the slant yellow ( positive reaction ) ( Lehman, 2005 ) . Some bacteriums are unable to ferment glucose, lactose, or sucrose. These bacteriums could deduce energy from peptones either aerobically or anaerobically ( Lehman, 2005 ) . Use of peptones causes the release of ammonium hydroxide ( NH3) ensuing in the pH index, phenol ruddy ( negative reaction ) , turning from pink to red ( Lehman, 2005 ) . Similar to the above, if the bacterium in inquiry used glucose ( dextroglucose ) so there would hold been little yellowing near where the bacterium was inserted due to formation of acid ( little yellowing due to little sum of glucose used – positive reaction ) ( Lehman, 2005 ) . But the angle would turn ruddy because the acid would respond with C dioxide and H2O in the ambiance ( Lehman, 2005 ) . Whereas if the angle was to go intensely ruddy ( for the same grounds as lactose use ) with no yellowing of the butt of where the angle was stabbed that would connote a negation reaction ( Lehman, 2005 ) . If the TSI angle were to go black in colour ( positive reaction ) this is because H2S is produced. This is because if the bacteriums can bring forth H2S ( which is colorless ) that H2S will respond with the Fe to organize Fe sulphide ( Lehman, 2005 ) . Note that sulfide decrease can merely take topographic point in an acidic environment, hence if H2S is produced some kind of agitation must hold taken topographic point ( Lehman, 2005 ) . Finally the last of the things that things that the TSI angle can observe for is the formation of gas. This can be determined by looking difficult to happen the formation air pockets ( positive reaction ) in the angle ( Lehman, 2005 ) . This occurs because of agitation bring forthing CO2which would seek to get away the angle but end up acquiring trapped ( Lehman, 2005 ) . For the presence of salmonella to be confirmed it would hold required a negative reaction of lactose/sucrose use, positive reaction of dextrose use, H2S production, and gas formation. Which for this experiment was non met. Potential beginnings of mistake could come from possible cross taint from airborne bacteriums, or the inoculating acerate leaf into the TSI angle.

Given the nature of this experiment these consequences would connote that the unknown # 2 given to us had the presence of coagulase positive strains of Staphylococci but no Salmonella in the beefburger sample provided. Both of these decisions have been found because both cheques for Staphylococci have been successful while both for Salmonella have non ( particularly given that the MacConkey agar sample was provided by the TA ) . Overall this was a successful experiment where the ends of placing if Salmonella and Stapylococcus Aureus were on the beefburger sample. Which in this instance merely Stapylococcus Aureus was found. Furthermore the other end was to reenforce research lab techniques which excluding the necessity to utilize the TA provided MacConkey agar due to our selenite cysteine stock traveling losing, was largely successful.


Table 3, Consequences from Enumeration and Enrichment of Staphylococcus – Raw settlement count


Colonies counted14400TMTC

Table 3 summarizes findings of experiment with regard to the figure of settlements found on the Stapylococcus 110 agar after incubation

Sample computations

Organisms/g = 14 * 106= 1.40 * 107

= 400 * 104= 4.00 * 106


Dinginesss, M, M. , Orwin, M, P. , and P, M Schlievert. 2000, Exotoxins of Staphylococcus aureus. Clinical Microbiology Review. Vol 13, pp 16-36

Foster, T. 1996, Staphylococcus, Medical Microbiology. 4th erectile dysfunction.

Hammack, T, S. , Amaguana, R, M. , June, G, A. , Sherrod, P, S. , Andrews, W, H.. 1999. Relative effectivity of selenite cystine stock, tetrathionate stock, and Rappaport-Vassiliadis medium for the recovery of Salmonella spp. from nutrients with a low microbic burden. Journal of nutrient protection. Vol 62, pp 16-21

Hardy, A. 2003. Salmonella: a go oning job, Postgraduate medical diary. Vol 80, Pp 541-545.

Lehman, D. 2005, Triple Sugar Iron Agar Protocols, American society for microbiology.

McGuckin, M, B, 1980, Screening for immune Gram-negative bacteriums with gentamicin-MacConkey agar. American Journal of Infection. Vol 8, p 85-86

Mitchell, C. ( 2010, November 28 ) . New Test Detects Salmonella Faster | Food Safety News. Retrieved June 16, 2015. & A ; lt ; hypertext transfer protocol: //www.foodsafetynews.com/2010/11/researchers-from-the-university-of/ & A ; gt ;

Smuckler, S. , and M, Appleman. 1964, Improved Stapylococcus medium No.110, Applied Microbiology. Vol 12, pp 355-359.

Department of Biology, 2015, Biology 241 Introduction to the microbic universe Laboratory manual. University of Waterloo, Waterloo. pp. 41-45.