Microbiology and Immunology

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MMacLeod, Colin MunroACLEOD, COLINMUNRO(1909-1972)

Canadian-born American microbiologist

Colin Munro MacLeod is recognized as one of the founders of

molecular biologyfor his research concerning the role of

deoxyribonucleic acid(DNA) in bacteria. Along with his col-

leagues Oswald Avery and Maclyn McCarty, MacLeod con-

ducted experiments on bacterial transformation which

indicated that DNA was the active agent in the genetic trans-

formation of bacterial cells. His earlier research focused on the

causes of pneumoniaand the development of serums to treat

it. MacLeod later became chairman of the department of

microbiology at New York University; he also worked with a

number of government agencies and served as White House

science advisor to President John F. Kennedy.

MacLeod, the fourth of eight children, was born in Port

Hastings, in the Canadian province of Nova Scotia. He was the

son of John Charles MacLeod, a Scottish Presbyterian minister,

and Lillian Munro MacLeod, a schoolteacher. During his child-

hood, MacLeod moved with his family first to Saskatchewan

and then to Quebec. A bright youth, he skipped several grades

in elementary school and graduated from St. Francis College, a

secondary school in Richmond, Quebec, at the age of fifteen.

MacLeod was granted a scholarship to McGill University in

Montreal but was required to wait a year for admission because

of his age; during that time he taught elementary school. After

two years of undergraduate work in McGill’s premedical pro-

gram, during which he became managing editor of the student

newspaper and a member of the varsity ice hockey team,

MacLeod entered the McGill University Medical School,

receiving his medical degree in 1932.

Following a two-year internship at the Montreal

General Hospital, MacLeod moved to New York City and

became a research assistant at the Rockefeller Institute for

Medical Research. His research there, under the direction of

Oswald Avery, focused on pneumonia and the Pneumococcal

infections which cause it. He examined the use of animal anti-

serums (liquid substances that contain proteins that guard

against antigens) in the treatment of the disease. MacLeod also

studied the use of sulfa drugs, synthetic substances that coun-

teract bacteria, in treating pneumonia, as well as how

Pneumococci develop a resistance to sulfa drugs. He also

worked on a mysterious substance then known as “C-reactive

protein,” which appeared in the blood of patients with acute

infections.

MacLeod’s principal research interest at the Rockefeller

Institute was the phenomenon known as bacterial transforma-

tion. First discovered by Frederick Griffith in 1928, this was a

phenomenon in which live bacteria assumed some of the char-

acteristics of dead bacteria. Avery had been fascinated with

transformation for many years and believed that the phenom-

enon had broad implications for the science of biology. Thus,

he and his associates, including MacLeod, conducted studies

to determine how the bacterial transformation worked in

Pneumococcal cells.

The researchers’ primary problem was determining the

exact nature of the substance which would bring about a trans-

formation. Previously, the transformation had been achieved

only sporadically in the laboratory, and scientists were not able

to collect enough of the transforming substance to determine its

exact chemical nature. MacLeod made two essential contribu-

tions to this project: He isolated a strain of Pneumococcus

which could be consistently reproduced, and he developed an

improved nutrient culturein which adequate quantities of the

transforming substance could be collected for study.

By the time MacLeod left the Rockefeller Institute in

1941, he and Avery suspected that the vital substance in these

transformations was DNA. A third scientist, Maclyn McCarty,

confirmed their hypothesis. In 1944, MacLeod, Avery, and

McCarty published “Studies of the Chemical Nature of the

Substance Inducing Transformation of Pneumococcal Types:

Induction of Transformation by a Deoxyribonucleic Acid

Fraction Isolated from PneumococcusType III” in the Journal

of Experimental Medicine.The article proposed that DNA was

the material which brought about genetic transformation.

Though the scientific community was slow to recognize the

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