Q.18 The bacterium that can tolerate high concentrations of salt and also ferment
mannitol is
(A) Staphylococcus aureus
(B) Staphylococcus epidermis
(C) Streptococcus pyogenes
(D) Serratia marcescens
Staphylococcus aureus is the bacterium that tolerates high salt concentrations and ferments mannitol, as demonstrated on Mannitol Salt Agar (MSA).
Option Analysis
Mannitol Salt Agar (MSA) contains 7.5% NaCl to select for salt-tolerant bacteria and mannitol with phenol red indicator to detect fermentation, which turns the medium yellow upon acid production.
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(A) Staphylococcus aureus: Grows on MSA due to high salt tolerance and ferments mannitol, producing yellow colonies with yellow zones from acid production.
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(B) Staphylococcus epidermidis: Grows on MSA as salt-tolerant but does not ferment mannitol, leaving red-pink colonies and no medium color change.
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(C) Streptococcus pyogenes: Inhibited by high salt in MSA, showing no or trace growth; lacks salt tolerance and mannitol fermentation.
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(D) Serratia marcescens: Ferments mannitol but generally inhibited by 7.5% NaCl as a Gram-negative bacterium, though some strains show limited growth.
Staphylococcus aureus stands out as the key bacterium high salt mannitol fermentation performer in microbiology labs, crucial for CSIR NET Life Sciences preparation. This pathogen thrives in selective media like Mannitol Salt Agar (MSA), making it a standard test for identification.
Mannitol Salt Agar Mechanism
MSA uses 7.5% NaCl to inhibit non-tolerant bacteria while allowing halotolerant species like staphylococci to grow. Mannitol fermentation by specific bacteria produces acid, shifting phenol red from red to yellow around colonies. This dual selective-differential action isolates bacterium high salt mannitol fermentation positives efficiently.
Bacterial Characteristics
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Staphylococcus aureus: Salt-tolerant with strong mannitol fermentation; yellow colonies confirm pathogenicity.
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Staphylococcus epidermidis: Salt-tolerant but mannitol-negative; red colonies differentiate it from aureus.
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Streptococcus pyogenes: Salt-sensitive; no growth on MSA.
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Serratia marcescens: Mannitol-positive but salt-intolerant Gram-negative; minimal or no growth.
| Bacterium | Salt Tolerance | Mannitol Fermentation | MSA Result |
|---|---|---|---|
| S. aureus | High | Positive (yellow) | Yellow colonies/zones |
| S. epidermidis | High | Negative | Red colonies |
| S. pyogenes | Low | Variable | No growth |
| S. marcescens | Low | Positive | Inhibited |
Exam Relevance
For CSIR NET aspirants, recognizing bacterium high salt mannitol fermentation via MSA distinguishes pathogenic staphylococci in clinical microbiology. Confirm positives with coagulase tests, as some strains vary. This knowledge integrates molecular biology, microbial physiology, and lab techniques essential for life sciences exams.
