An important cause of tooth decay by the bacterium Streptococcus mutans is because of their ability to attach to teeth surface by producing slime layer from
- A.Sugar
- B.Alcohol
- C.Saliva
- D.Teichoic acid
Correct answer
Explanation
The correct answer is A, sugar. Streptococcus mutans turns dietary sugar, especially sucrose, into a sticky slime that glues it to the teeth.
The bacterium makes enzymes called glucosyltransferases that join the glucose units of sucrose into sticky chains called glucans. These help the bacteria cling to the enamel and build dental plaque. Inside the plaque the bacteria ferment more sugar into lactic acid, and when the pH at the tooth surface falls below about 5.5 the enamel begins to dissolve, which is how a cavity starts. This is why frequent sugary snacks harm teeth more than the same sugar eaten at once.
B, alcohol, is not what the bacterium builds its slime from.
C, saliva, actually protects teeth by washing away food and neutralising acid.
D, teichoic acid, is part of the cell wall of Gram-positive bacteria, not the raw material of the slime layer.
Exam tip: Streptococcus mutans plus sucrose gives sticky glucans and lactic acid; enamel dissolves below pH 5.5.
Practice Questions
View allWhich one of the following plant tissues has dead cells ?
- A.Epidermis
- B.Parenchyma
- C.Collenchyma
- D.Sclerenchyma
Show answer
Sclerenchyma cells are dead at maturity, with thick walls hardened by lignin. Sclerenchyma is the tissue that makes plants hard and stiff.
Read the full explanation →Imagine a current-carrying straight conductor with magnetic field of lines in anti-clockwise direction. Then the direction of current is determined by
- A.the Right-Hand Thumb rule and it would be in the downward direction.
- B.the Left-Hand Thumb rule and it would be in the downward direction.
- C.the Right-Hand Thumb rule and it would be in the upward direction.
- D.the Left-Hand Thumb rule and it would be in the upward direction.
Show answer
For a straight wire, the right-hand thumb rule links the current to the circular field lines around it. Hold the wire in the right hand with the thumb along the current; the curled fingers then show the direction of the magnetic field lines, which form concentric circles around the wire.
Read the full explanation →Which one of the following statements is true for sound waves propagating in air?
- A.Sound is an electromagnetic wave and transverse in nature
- B.Sound is a mechanical wave and longitudinal in nature
- C.Sound is a mechanical wave and transverse in nature
- D.Sound is an electromagnetic wave and longitudinal in nature
Show answer
Sound needs a material medium to travel, and in air its particles vibrate back and forth along the direction the wave moves.
Read the full explanation →Which one of the following is primarily responsible for conduction of current in a metal?
- A.Bound electrons
- B.Free electrons
- C.Both bound and free electrons
- D.Ions
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In a metal, the outer electrons of the atoms are not tied to any one atom and drift through the lattice when a voltage is applied, carrying the current.
Read the full explanation →Which one of the following statements is correct about the magnification of an optical microscope?
- A.Magnification increases with the increase in focal length of eyepiece
- B.Magnification increases with the increase in focal length of objective
- C.Magnification does not depend upon the focal length of eyepiece
- D.Magnification decreases with the increase in focal length of eyepiece
Show answer
A compound microscope magnifies more when both its lenses have short focal lengths. In a compound microscope the objective forms a real, enlarged image, and the eyepiece magnifies it further like a simple magnifier.
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