Name: ______Date: ______Period: ______

Identifying Unknown Bacteria

Text reference: CCP (macaw) pp.580-583; CP(creepy eye) pp. 362-365; Elephant book pp. 363-367.

Introduction and background

Just seeing an individual bacterium requires a very good light microscope with 1000x magnification. Even when you manage to see them, you will find that there are only a handful of appearance differences that can be noticed, yet hundreds of different species of bacteria are known. How then does one figure out what an unknown bacterium is?

Microbiologists have found that using a logical series of tests and observations of a bacterial sample allows for the determination of the particular species possible through the process of elimination. The table below gives characteristics for a variety of bacteria that are often encountered in a medical laboratory. A plus sign (+) in the table means that the organism has that particular characteristic. A minus sign (-) means the organism lacks that characteristic. A blank means the information is not known.

Sugars fermented
Organism / Associated disease / Shape or form / Type of cell wall / Capsule / Flagella / Endospores / Aerobic / Causes lysis of red blood cells / Grows well at 20oC / glucose / maltose / lactose
Bacillus anthracis / anthrax / bacillus / A / + / - / + / + / - / - / + / + / +
Bacillus subtilis / generally none / bacillus / A / - / - / + / + / - / + / + / + / +
Clostridium botulinum / botulism / bacillus / A / - / + / + / - / + / - / + / -
Clostridium tetani / tetanus / bacillus / A / - / + / + / - / + / - / - / -
Diplococcus pneumoniae / bacterial pneumonia / diplococcus / A / + / - / - / + / + / -
Escherichia coli / food poisoning / bacillus / B / - / - / - / + / - / - / + / + / +
Klebsiella pneumoniae / urinary infection / bacillus / B / + / - / - / + / - / - / + / + / +
Neisseria catarrhalis / respiratory infection / diplococcus / B / - / - / - / + / - / + / + / + / -
Neisseria gonorrheae / gonorrhea / diplococcus / B / - / - / - / + / - / - / + / - / -
Neisseria meningitidis / bacterial meningitis / diplococcus / B / - / - / - / + / - / - / - / - / -
Pseudomonas aeruginosa / opportunistic; resistant / bacillus / B / - / + / - / + / - / + / - / - / -
Salmonella typhi / typhoid fever / bacillus / B / - / + / - / + / - / + / + / -
Serratia marcescens / opportunistic; resistant / bacillus / B / - / + / - / + / + / + / + / -
Staphylococcus aureus / skin, TSS; resistant / staphylococcus / A / - / - / - / + / + / - / +
Staphylococcus epidermidis / generally none / staphylococcus / A / - / - / - / + / - / - / +
Streptococcus fecalis / cardiac, urinary; resistant / streptococcus / A / - / - / - / + / - / - / +
Streptococcus hemolyticus / strep throat, scarlet fever, TSS, NF / streptococcus / A / - / - / - / + / + / - / +
  1. List the four possible shapes and growth patterns of the bacteria listed in the chart on the reverse of this sheet. Describe these shapes and patterns in words. Note: there are only four different growths, it is not necessary to duplicate answers.
  1. What test is used to determine “cell wall type A” or “cell wall type B”?
  1. What is actually different about cell wall types A and B?
  1. (The stumper) None of the bacteria that lyse (burst) red blood cells grow well at 20oC. What temperature do they likely grow best at? Explain your reasoning.

Identify the unknown bacteria.

  1. Unknown A is examined under a microscope and observed to be a diplococcus. A test is done that shows the diplococcus has cell wall type B and has no capsule. Samples of unknown A are placed in incubators at 20oC and at 37oC. The sample at 37oC grows well, but the one at 20oC does not. The unknown is tested for the ability to ferment the sugars glucose, maltose and lactose. It can only ferment glucose.

The identity of unknown A is ______.

  1. Unknown B is a bacillus with cell wall type A. It is anaerobic and causes lysis (bursting) of red blood cells when grown in a blood culture. This organism is tested for the ability to ferment the sugars glucose and lactose. It cannot ferment either of these sugars.

The identity of unknown B is ______.