3. Why Do The Different Plates, Differential And Selective, Use Tsa As The Background Media?
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ABSTRACT
The genus Bacillus is a group of gram-positive endospore-forming bacteria that can cause food poisoning and diarrheal illness in humans. A wide range of food products have been linked to foodborne outbreaks associated with these opportunistic pathogens. The U.S. Food and Drug Assistants recommends (in their Bacteriological Belittling Manual) the utilize of Bacara or mannitol egg yolk polymyxin (MYP) agar plates and the nearly-likely-number (MPN) method for enumeration and confirmation of Bacillus cereus and related species isolated from foods, desultory cases, outbreaks, and routine environmental surveillance samples. We performed a comparative analysis of two chromogenic media (Bacara and Luminescence) and two traditional media (MYP and polymyxin egg yolk mannitol bromothymol blue agar [PEMBA]) for the isolation and enumeration of xvi Bacillus species nether modified growth conditions that included pH, temperature, and dilution factor. A total of 50 environmental, food, and American Type Culture Collection reference isolates from 16 distinct Bacillus species were evaluated. A food adulteration experiment also was carried out by artificially adulterating 2 baby food matrices with two isolates each of B. cereus and Bacillus thuringiensis. Our results clearly indicated that chromogenic plating media (Bacara and Brilliance) are meliorate than conventional standard media (MYP and PEMBA) for the detection and enumeration of B. cereus in foods and other official regulatory samples. The comparison of the ii chromogenic media also indicated that Brilliance medium to exist more efficient and selective for the isolation of Bacillus.
The genus Bacillus comprises rod-shaped, gram-positive, endospore-forming bacterial species that can be obligate aerobes or facultative anaerobes. Even so, the majority of Bacillus species are aerobic, are known to contaminate raw and processed foods, and crusade foodborne outbreaks and nutrient poisoning in humans. The taxonomy of Bacillus is complex; in the past, 145 species have been considered function of this genus (13). Still, only 40 species are currently recognized equally Bacillus and listed in Bergey's Manual of Systematic Bacteriology (half dozen). Bacillus cereus is a pathogenic foodborne species causing mild to severe nausea, vomiting, and diarrheal illness in humans (viii, 12, 18, 19). This species also tin cause several nongastrointestinal diseases such equally systemic infections, middle infections, and periodontitis (ix, x, 12, 27). Bacillus thuringiensis is a known biological insecticide that has also been linked to foodborne illnesses and outbreaks (ane, 17). Several other Bacillus species, including B. licheniformis, B. subtilis, B. pumilus, likewise have been reported every bit causing foodborne illness in humans (17).
Initially, the B. cereus grouping was divers equally containing 3 species: B. cereus, B. thuringiensis, and B. anthracis (iv, 25). Although these species were considered genetically similar, B. anthracis was believed to exist a monophyletic clone derived from B. cereus and B. thuringiensis (two, 4, vii, 24, 25). More recently, 6 closely related species have been included in the B. cereus group: B. cereus, B. anthracis, B. thuringiensis, B. mycoides, B. pseudomycoides, and B. weihenstephanensis (33). Notwithstanding, genetic discrimination inside the B. cereus group has been a challenging task at both the inter- and intraspecies level. Some fence has been focused on whether B. cereus, B. anthracis, and B. thuringiensis are varieties of the same species, based on the controversial results of chromosome analysis and toxin capsule data (15).
Data from agile and passive surveillance and other sources during 2000 through 2008 was used to gauge that approximately 9.four million foodborne illnesses were caused by a known pathogen each year in the U.s., with 55,961 hospitalizations and 1,351 deaths (11). Of these illnesses, 1.3 one thousand thousand (14%) were caused past B. cereus, Clostridium perfringens, or Staphylococcus aureus. In a separate study, i,229 foodborne outbreaks caused by B. cereus, C. perfringens, and Due south. aureus were reported in the United States from 1998 to 2008, and only 39% of these outbreaks had a confirmed etiology (3).
Two standard bacterial media, mannitol egg yolk polymyxin (MYP) and polymyxin egg yolk mannitol bromothymol bluish agar (PEMBA), have been recommended past the U.S. Food and Drug Administration (FDA) and several other regulatory agencies for recovery of Bacillus from diverse food matrices (31). Withal, several limitations of these media have been described, including the generation of colonies with atypical morphology and the production of lecithinase, that ultimately mask Bacillus growth by allowing growth of groundwork flora (12, 30, 32). Thus, these limitations may lead to wrong identification of members of the B. cereus group (10, 29).
In recent years, the development of chromogenic media has led to improved accuracy and rapid detection of a wide range of human pathogenic leaner of public health importance. Chromogenic media too have an advantage over traditional bacterial media because occasionally isolation in pure culture and confirmatory tests may not be required (12, 21, 22, 26). In a recent study, the growth and phenotypic variation of Cronobacter was evaluated under modified civilisation atmospheric condition, and pocket-sized variations in growth weather condition contradistinct colony morphology and the expression of unique biological characteristics, and these phenotypic changes may be useful for differentiating various Cronobacter strains (28).
In the nowadays report, we evaluated the efficacy of some traditional (MYP and PEMBA) and chromogenic (Bacara and Luminescence) bacterial media for optimal recovery of B. cereus and its closely related species. A full of 50 isolates belonging to sixteen Bacillus species were examined under modified growth weather condition considering rapid isolation of human being pathogenic bacteria is imperative for precise identification of isolates recovered during investigation of foodborne outbreaks, sporadic cases, consumer complaints, and routine surveillance samples. A nutrient adulteration experiment too was carried out by artificially infecting two baby nutrient matrices with iv isolates of B. cereus and B. thuringiensis.
MATERIALS AND METHODS
Bacterial strains
In this study, 50 isolates belonging to 16 species of Bacillus were used: 14 isolates of B. cereus; five isolates of B. thuringiensis; iii isolates each of B. circulans, B. coagulans, B. licheniformis, B. megaterium B. pumilus, B. subtilis, and B. vallismortis; 2 isolates each of B. lentus, B. mycoides, and B. sphaericus; and one isolate each of B. amyloliquefaciens, B. firmus, B. laterosporus, and B. smithii (Table i). Of these 50 isolates, 17 were American Type Civilization Collection (ATCC) reference cultures. Other reference isolates of C. perfringens (ATCC 13124), Escherichia coli (ATCC 25922), Listeria monocytogenes (ATCC 19115), S. aureus (ATCC 6538), and Salmonella enterica subsp. enterica serovar Gaminara (ATCC 8324) were also used as controls throughout evaluation and optimization in the growth condition experiments.
Table ane.
Bacterial levels and colony morphology characteristics (CMC) of 16 Bacillus species on v media
Culture and selective plating media
The Bacillus isolates were retrieved from frozen glycerol stock cultures by transferring a loopful (approximately 10 μL) to x mL of brain eye infusion broth (BHI) at pH 7.2 ± 0.2 (BD, Sparks, MD) and incubating overnight at 35.5 ± 2°C. Subsequently, 100 μL of the BHI culture was streaked onto a food agar (NA) plate and incubated at 35.5 ± 2°C for 24 h. A single colony from each isolate was used to inoculate 10 mL of BHI, which was incubated overnight at 35.5 ± 2°C. Serial dilutions from 10−i to 10−half-dozen were made from the single jail cell culture, and 100 μL of each culture was streaked onto NA plates in triplicate and incubated at 35.5 ± ii°C for 24 ± 2 h to determine the optimal dilution to exist used for bacterial counts. Trypticase soy agar (TSA) plates were also utilized in some cases (Table ane). Each isolate was then streaked in triplicate onto plates of two selective media (PEMBA, HiMedia, Bombay, India; MYP, Neogen/Acumedia, Lansing, MI) and ii chromogenic media (Bacara, bioMérieux, Hazelwood, MO; Brilliance, Oxoid, Basingstoke, UK) following the manufacturers' protocols. The MYP and Bacara plates were incubated at thirty ± ii°C for sixteen to 24 ± 2 h, and PEMBA and Brilliance plates were incubated at 37.5 ± ii°C for 24 ± 2 h.
Colony morphology, detection, and enumeration
The growth characteristics were recorded later overnight incubation for each media. If no growth was observed after 24 ± 2 h, the incubation was connected for upward to 96 ± two h, and the plates were checked every 24 ± 2 h for whatever growth (Table 1).
Upshot of temperature on growth over fourth dimension and growth measurement
To sympathise the effect of temperature on growth, single cell cultures were grown for each of the l isolates of the 16 Bacillus species. Each civilisation was inoculated into BHI (pH vii.ii ± 0.2) in triplicate and incubated at iv temperatures: room temperature (RT; 4 ± 1°C), 24 ± 2°C, 35.5 ± 2°C, and fifty ± two°C. Cultures were shaken at 100 rpm on a rotary shaker for 72 ± ii h. To estimate the total number of colonies recovered from 1 mL of the original broth culture, the CFU per milliliter was calculated using this formula: CFU per 0.ane mL × dilution cistron. The optical density at 600 nm of the BHI culture was determined by analyzing 1 mL of culture with a BioPhotometer Plus (AG 22331, Eppendorf, Hamburg, Germany) after 24 ± 2 h of growth.
Result of pH on growth over fourth dimension and growth measurement
To assess the effects of pH on the growth of 16 Bacillus species, the l isolates were cultured in BHI (pH seven.2 ± 0.two) under four pH atmospheric condition: eight.ii ± 0.2, 7.two ± 0.2, half-dozen.2 ± 0.2, and five.2 ± 0.2. For each isolate, the previously determined optimal dilution and growth temperature were besides applied every bit described in Table i. The inoculated BHI cultures with the four pH values were grown in triplicate and incubated at previously determined optimal temperatures (as described in Table 1) with shaking at 100 rpm on a rotary shaker for 72 ± 2 h. The optical density at 600 nm of the BHI culture was determined past analyzing 1 mL of civilization with the BioPhotometer Plus afterwards 24 ± 2 h of growth.
Food spiking experiment
A food cariosity experiment was carried out by artificially contaminating two infant food matrices (unmarried grain rice cereal and unmarried grain oatmeal cereal, Gerber, Fremont, MI) with two isolates each of B. cereus (ATCC 11778 and S-684684) and B. thuringiensis (ATCC 33679 and S-761007), following a previously described method (12). For each set up of experiments, 25.0 g of the food sample was homogenized with 225.0 mL of BHI supplemented with 0.1% sucrose in triplicate and so spiked with 100 μL of the appropriate freshly prepared unmarried jail cell culture (Table 1). The negative controls were also grown with (i) 25.0 g of food in BHI and (ii) 225.0 mL of BHI lone; both cultures were incubated at 37.5 ± 2°C for 24 ± 2 h. Serial dilutions of the spiked food matrices and the negative controls were then streaked in triplicate onto MYP, PEMBA, Bacara, and Luminescence plates. The Brilliance and PEMBA plates were incubated at 37.5 ± ii°C and the Bacara and MYP plates were incubated at 30.0 ± 2 °C for 12 to 24 h.
RESULTS AND DISCUSSION
Colony morphology, detection, and enumeration
In this study, 50 isolates from 16 Bacillus species were examined: 17 ATCC reference isolates, xv food isolates, fifteen environmental isolates, and 3 isolates from corrective products. For the optimization of colony counts, the NA cultures were used virtually oft, and bacterial growth was observed after 24 to 48 ± 2 h of incubation at 35.5 ± 2°C (Table one). TSA cultures also were used in some cases in which no growth was observed on NA plates (Table 1).
A longer incubation time (48 to 72 ± 2 h) was needed for some of the isolates of B. amyloliquefaciens, B. circulans, B. coagulans, B. laterosporus, B. lentus, B. pumilus, B. smithii, B. sphaericus, and B. vallismortis. The BHI was suitable for the initial civilization of all Bacillus species except some isolates of B. circulans and B. megaterium, which grew better in Trypticase soy broth. Growth was observed in BHI afterward incubation for 48 h at 35.5 ± 2°C for three isolates of B. pumilus and B. coagulans and 1 isolate each of B. amyloliquefaciens and B. vallismortis. However, subsequent streaking on NA required 24 to 48 ± two h of incubation at l ± two°C for colony germination and counting (Table 1).
Effects of temperature on the growth of Bacillus spp
To understand the effects of temperature on the growth, the l isolates of sixteen Bacillus species were cultured at iv temperatures (RT, 24 ± 2°C, 35.five ± 2°C, and 50 ± two°C) in BHI at pH 7.ii ± 0.two (Table 2). The impact of temperature on the growth of all sixteen Bacillus species was axiomatic. No growth was observed at the everyman temperature (4 ± 2°C) in any of the Bacillus isolates tested, and a significantly lower growth was noticed at the highest temperature in nigh cases (Table ii). In general, growth was observed at both RT and 35.5 ± 2°C later on 24 ± ii h of incubation, and no species-specific trend was noted. The unmarried variable ANOVA was performed to compare the divergence between growth at the 4 temperatures, and a significant difference (P < 0.05) was found. Thus, optimal growth can be achieved at room temperature and 35.5 ± 2°C for most of the isolates of these xvi Bacillus species (Table 2).
TABLE 2
Result of temperature on the growth of 16 Bacillus species at ten−v dilutiona
Effects of pH on the growth of Bacillus spp
The furnishings of pH on the growth of these Bacillus isolates were evaluated with previously determined advisable amounts of culture (Table 1) and incubation conditions at two temperatures (RT and 35.five ± 2°C) for 24 ± 2 h (Table 2). Iv pH weather condition (8.2 ± 0.2, 7.2 ± 0.2, 6.2 ± 0.ii, and 5.two ± 0.2) were tested for the optimal growth of the Bacillus isolates in BHI culture with constantly shaking at 100 ± two rpm on a rotary incubator shaker. The single variable ANOVA was performed to compare the differences in growth nether the iv pH atmospheric condition. No significant species-specific tendency was found amid the four pH conditions (Table 3).
TABLE 3
Effect of pH on the growth of 16 Bacillus species at 10−5 dilutiona
Evaluation of traditional differential and selective chromogenic media on the growth of Bacillus spp
In this study, two nonselective traditional bacterial media (MYP and PEMBA) and 2 selective chromogenic bacterial media (Bacara and Brilliance) were tested for the optimal recovery of Bacillus spp. A full of 38 isolates of 16 Bacillus species and five isolates of non-Bacillus species were grown on these four plating media to evaluate whatever differences in sensitivity and selectivity. Although B. cereus and B. thuringiensis isolates grew on all 4 media, distinct differences in growth patterns were observed for the other Bacillus species and the v non-Bacillus species on these media (Table 1).
A like growth blueprint was observed for Bacillus isolates plated on MYP and PEMBA, except that the colonies appeared pink or orange-pink on MYP and bluish or bluish dark-green on PEMBA after 24 to 48 ± 2 h of incubation in almost cases (Figs. one and 2). Most (11) of the Bacillus species grew on both of these media: B. cereus, B. thuringiensis, B. amyloliquefaciens, B. mycoides, B. laterosporus, B. lentus B. licheniformis, B. pumilus, B. sphaericus, B. subtilis, and B. vallismortis. Isolates of iv other species, B. coagulans, B. firmus, B. megaterium, and B. smithii, did non grow on either of these media, and the isolate of B. circulans grew only on MYP.
FIGURE 1
Colony morphology of Bacillus spp. isolates grown on MYP traditional medium. (A) B. amyloliquefaciens 171; (B) B. cereus 97; (C) B. circulans 27; (D) B. laterosporus 36; (E) B. lentus 318; (F) B. licheniformis 13; (Thousand) B. mycoides 319; (H) B. pumilus 32; (I) B. sphaericus 20; (J) B. subtilis 252; (G) B. thuringiensis 9; (L) B. vallismortis 203.
FIGURE 1
Colony morphology of Bacillus spp. isolates grown on MYP traditional medium. (A) B. amyloliquefaciens 171; (B) B. cereus 97; (C) B. circulans 27; (D) B. laterosporus 36; (E) B. lentus 318; (F) B. licheniformis 13; (K) B. mycoides 319; (H) B. pumilus 32; (I) B. sphaericus twenty; (J) B. subtilis 252; (1000) B. thuringiensis 9; (50) B. vallismortis 203.
Close modal
FIGURE ii
Colony morphology of Bacillus spp. isolates grown on PEMBA traditional medium. (A) B. amyloliquefaciens 171; (B) B. cereus 97; (C) B. cereus 209; (D) B. laterosporus 36; (E) B. lentus 318; (F) B. licheniformis 164; (G) B. mycoides 319; (H) B. pumilus 32; (I) B. sphaericus twenty; (J) B. subtilis 252; (Thousand) B. thuringiensis 7; (L) B. vallismortis 203.
Effigy 2
Colony morphology of Bacillus spp. isolates grown on PEMBA traditional medium. (A) B. amyloliquefaciens 171; (B) B. cereus 97; (C) B. cereus 209; (D) B. laterosporus 36; (E) B. lentus 318; (F) B. licheniformis 164; (M) B. mycoides 319; (H) B. pumilus 32; (I) B. sphaericus twenty; (J) B. subtilis 252; (K) B. thuringiensis 7; (50) B. vallismortis 203.
Shut modal
Of the xvi Bacillus species plated on the chromogenic Bacara medium, only 6 grew: B. cereus, B. thuringiensis, B. mycoides, B. lentus, B. licheniformis, and B. sphaericus. Uniform private pink-orange colonies were observed for B. cereus, B. thuringiensis, B. mycoides, and B. lentus inside 18 to 24 ± 2 h of incubation (Fig. 3). However, pink pinpoint watery colonies without a zone of precipitation were noticed for B. licheniformis and ane isolate of B. sphaericus subsequently 48 to 96 ± 2 h of incubation. Of the five not-Bacillus species plated on Bacara, pink colonies without precipitation were apparent for S. enterica, S. aureus, L. monocytogenes, and C. perfringens, and no growth was observed for E. coli.
FIGURE 3
Colony morphology of Bacillus spp. isolates grown on Bacara chromogenic medium. (A) B. cereus 119; (B) B. lentus 318; (C) B. licheniformis 164; (D) B. mycoides 319; (E) B. sphaericus 20; (F) B. thuringiensis 246.
Effigy 3
Colony morphology of Bacillus spp. isolates grown on Bacara chromogenic medium. (A) B. cereus 119; (B) B. lentus 318; (C) B. licheniformis 164; (D) B. mycoides 319; (E) B. sphaericus 20; (F) B. thuringiensis 246.
Shut modal
When isolates of the 16 Bacillus species were plated on Luminescence medium, only four species, B. cereus, B. thuringiensis, B. mycoides, and B. lentus, grew after 12 to 24 ± 2 h of incubation, and the colonies were characteristic green or bluish light-green or turquoise green with a distinct precipitation zone. The balance of the Bacillus species and the five non-Bacillus isolates also did not grow on Luminescence medium despite longer incubation times of up to 96 h (Fig. 4).
Figure four
Colony morphology of Bacillus spp. isolates grown on Brilliance chromogenic medium. (A) B. cereus 52; (B) B. lentus 318; (C) B. mycoides 319; (D) B. thuringiensis 8.
FIGURE 4
Colony morphology of Bacillus spp. isolates grown on Brilliance chromogenic medium. (A) B. cereus 52; (B) B. lentus 318; (C) B. mycoides 319; (D) B. thuringiensis eight.
Shut modal
The results of this report signal that the two chromogenic media (Bacara and Brilliance) were more than selective than the traditional media (MYP and PEMBA), and characterization and enumeration of colonies was easier on the chromogenic media. The traditional selective media (MYP and PEMBA) produced similar results for isolates of all sixteen Bacillus species. On MYP and PEMBA, 11 species (B. cereus, B. thuringiensis, B. amyloliquefaciens, B. laterosporus, B. lentus, B. licheniformis, B. mycoides, B. pumilus, B. sphaericus, B. subtilis, and B. vallismortis) produced colonies with typical morphology, with a zone of atmospheric precipitation on MYP and a lecithinase-positive reaction on PEMBA. The colonies were pink or orange-pink on MYP and peacock blue or greenish blue on PEMBA equally expected. The only isolate of B. circulans tested on these two traditional media grew on MYP, but growth was inhibited on PEMBA. The growth of the remaining Bacillus isolates (B. coagulans, B. megaterium, B. smithii, and B. firmus) was inhibited on both MYP and PEMBA. All isolates of the gram-negative and gram-positive non-Bacillus species except L. monocytogenes grew on MYP and PEMBA. Only four Bacillus species (B. cereus, B. thuringiensis, B. mycoides, and B. lentus) grew on Brilliance medium, just 6 species (B. cereus, B. thuringiensis, B. mycoides, B. lentus, B. licheniformis, and B. sphaericus) grew on Bacara medium. When these ii chromogenic media were tested with isolates of five gram-positive and gram-negative non-Bacillus species, Brilliance was more selective than Bacara; none of the v species grew on Luminescence, simply 4 (Fifty. monocytogenes, C. perfringens, S. enterica, and S. aureus) grew well on Bacara (Tabular array ane).
The selective growth blueprint of chromogenic plating media has been attributed to the synthetic chromogenic substrates broken past specific enzymatic activities for related microorganisms (20, 21, 26). The chromogenic substrate v-bromo-4-chloro-3-indolyl-β-glucopyranoside in Brilliance is broken by enzyme β-glucosidase present in B. cereus, resulting in the formation of light-green or light-green-turquoise colonies with whitish centers. Because B. thuringiensis is biochemically identical to B. cereus, these species produce colonies with the same morphology. Brilliance also contains two antibiotics (polymyxin B and trimethoprim) that inhibit the growth of most gram-negative and gram-positive bacteria, including S. aureus, which is often misdiagnosed during B. cereus detection and differentiation. In several studies, advantages of chromogenic media over traditional media accept been noted for sensitivity, selectively, characteristic colony morphology, isolation, and enumeration of B. cereus. The ii conventional selective media (MYP and PEMBA) accept been compared with the same ii chromogenic plating media used here: Brilliance (v, 12, 14, 16) and Bacara (23, 30). The more selective results obtained with the Brilliance medium is perchance associated with the inclusion of trimethoprim in this medium (5) and its inhibitory action against non-Bacillus species compared with that of conventional media (12). Although trimethoprim inhibits the growth of only gram-positive bacteria, the addition of polymyxin B in the Brilliance medium results a synergistic result besides inhibiting growth of gram-negative bacteria (26). Growth inhibition of some B. cereus isolates was noticed on Brilliance medium, whereas these isolates did grow on MYP medium (12, fourteen). Results of this study on the comparison of chromogenic media (Bacara and Brilliance) versus traditional media (MYP and PEMBA) are in complete agreement with results of previous studies (5, 12, 21, 23, thirty).
Nutrient spiked with four isolates of B. cereus and B. thuringiensis
In the nutrient adulteration studies using two food matrices spiked with iv isolates of B. cereus and B. thuringiensis, no significant differences were observed in the recovery of these organisms on the four selective media tested (Table 4). Although all four media produced colonies with morphology typical of B. cereus and B. thuringiensis from cultures of the spiked food samples, the differentiation and enumeration of both Bacillus species on MYP and PEMBA were significantly inhibited past low levels of competing microorganisms. The chromogenic Bacara and Brilliance media generated more identifiable colonies than did MYP and PEMBA because these chromogenic media inhibited the groundwork microflora, improving enumeration and isolation of B. cereus and B. thuringiensis.
Table 4
Colony counts of B. cereus and B. thuringiensis in the artificially contaminated food samples based on nine readings of iv dilutions plated in triplicate on four mediaa
In summary, the results of the present study advise that the two chromogenic media (Bacara and Luminescence) were more selective than the two traditional media (MYP and PEMBA). Analysis of various growth characteristics and enumeration for xvi Bacillus species and 5 non-Bacillus species on these 4 plating media indicated that the characterization and enumeration of colonies was easier on the chromogenic media than on the traditional nonchromogenic media. The Brilliance chromogenic plating medium was better than the Bacara medium for differentiating B. cereus and B. thuringiensis from the non-Bacillus species. Our findings also suggest that Brilliance medium could be used for the recovery of these two important bacteria (B. cereus and B. thuringiensis) from various official samples associated with cases of foodborne diseases and can help our agency accomplish its mission of protecting the public health by assuring that foods are safe for consumption.
ACKNOWLEDGMENTS
The findings and conclusions in this article are those of the authors and practise not necessarily represent the views or official position of the FDA. The names of vendors or manufacturers are provided as examples of available product sources; inclusion does not imply endorsement of the vendors, manufacturers, or products by the FDA or the U.S. Section of Health and Human Services. This study was supported in part past funding from the FDA Commissioner'due south Fellowship Programme. The funders had no role in the study design, data collection and analysis, decision to publish, or training of the commodity.
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3. Why Do The Different Plates, Differential And Selective, Use Tsa As The Background Media?,
Source: https://meridian.allenpress.com/jfp/article/80/6/952/200015/Evaluation-of-Two-Standard-and-Two-Chromogenic
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