A strain of pneumococcus bacteria eight times more resistant to penicillin than any before seen in the United States has been identified in a 5-year-old girl in Minnesota.

The child's illness has been successfully treated by another antibiotic - one more dangerous to use than penicillin - at the University of Minnesota Hospitals in Minneapolis.

But the finding tells doctors that the germs that cause most bacterial pneumonia may be turning increasingly resistance to the drug that has been the best weapon against this often fatal illness.

The girl's case was reported in the Morbidity and Mortality Weekly Report of teh federal Center for Disease Control in Atlanta yesterday.

"It's another example of the emerging resistance of many bacteria to many drugs," Dr. Robert Austrian of the University of Pennyslvanis, a leading authority on the pneumococcus germ, said in and interview.

"It's a scary thing," said Dr. Paul Quie, professor of pediatrics at the Minnesota university. "Here is and organism that has been uniformly sensitive to pencillin for many years.

"This girl was a very special case, it's true. Because of a rare immunological deficiency that has caused her to have many infections, she has been getting penicillin prophylactically" - as a preventive - "for years. This would naturally give resistant strains of the germ a chance to develop.

"But any time a resistance like this appears, one fears it can happen more often," he said. In recent weeks:

Alarmed doctors in South Africa have reported finding a deadly new strain of pneumococcus resistant not only to penicillin but to most antibiotics.

Doctors are continuing to find sporadic cases in the United States 210 in all since March, 1976, of a highly penicillin-resistant strain of Asian gonorrhea germ that now represents about 30 per cent of all gonorrhea strains in the Philippines.

The Food and Drug Administration has said that, to help prevent new resistant bacteria from developing, it will ban the current wide use in animal feeds of some antibiotics. FDA Commissioner Donald Kennedy said the first drug banned will be penicillin, but his final ruling may be nearly a year off, while sales of the feeds continue.

"The pneumococcus organism had been a relative laggard in developing resistance," Dr. Austrian said. As Dr. Quie put it, "It has been singularly gentlemanly compared to othr organisms."

Dr. Joel Ward of CDC, just back from South Africa to investigate the problem in the country, reported that the organism has so far caused 15 cases of serious blood defects or meningitis (brain inflammation), with five deaths. Another 300 persons have been found to be carriers.

"We're much more worried about the South African findings than about the Minnesota case," Ward said. Alto, some "partially penicillin-resistant" strains of pneumococci have recently been found in Canada and Australi, he reported.

"What's important," he said, "is that people understand that bacteria in general are rapidly becoming resistant to antibiotics. People should only take antibiotics under a physician's supervision, and physician should only prescribe them for suspected or proved bacterial infections, not for viral infections."

Antibiotics are ineffective against viruses. But many doctors prescribe them for colds and other respiratory illnesses caused by viruses - usually, they say, to combat any "bacterial complications." Doctors confronted with sick patients often use large doses of antibiotics before they are sure what disease they are treating.

"Sad to say, there is over-use of antibiotics," said Dr. William Shaffner of Vanderbilt University, who has studied some prescribing practicesin Tennessee.

"Shotgun therapy" - quick use of the most likely-seeming drug - is sometimes needed to attack hard-to-diagnose illnesses, Dr. Ward conceded. "But if it's overdone," he said, "I'm afraid we may suffer some long-term consequences for only hsort-term gains."