Alexander C. Sonnenwirth was born in Oradea, Romania into a German-speaking Jewish family. In addition to German, Sonnenwirth learned Romanian, Hungarian, and Hebrew as a child. After completing his secondary education, Sonnenwirth went to Budapest to stay with relatives while he worked as a photographer. However, World War II shattered the world in which he and his family lived. Most of the Jews of Oradea, including Sonnenwirth's parents, were sent to death camps by the German invaders. Sonnenwirth escaped that fate, but was forced to serve in a labor gang for the duration of the war until he was rescued by Allied forces.
Immediately after the war, Sonnenwirth lived in a camp for displaced persons in Marburg, Germany. He was awarded a Hillel Scholarship which enabled him to come to the United States to study bacteriology at the University of Nebraska. After earning a Bachelor's degree in 1950, Sonnenwirth continued his studies at Purdue University where he graduated with a Master's of Science in 1953. While a student, he married Rosaline Soffer, and in 1953, the Sonnenwirths moved to St. Louis when he was appointed Assistant Director of the Division of Bacteriology at Jewish Hospital.
Sonnenwirth became the director of the division in 1955 and began doctoral studies in bacteriology at Washington University. Studying under Dr. Theodore Rosebury of the School of Dentistry, Sonnenwirth received his PhD in 1960. In addition to his duties at Jewish Hospital, Sonnenwirth served several academic appointments including Instructor of Bacteriology in the School of Dentistry (1958-1961) and as Assistant Professor in the School of Medicine for the Departments of Microbiology (1962) and Pathology (1968). In 1970, he was promoted to Associate Professor in the latter two departments and became a full Professor in 1977.
Sonnenwirth's scientific contributions included both 'pure' research and innovation in clinical technology. His chief research specialty was the study of anaerobic gram-negative bacilli. His enormous knowledge in this and related fields was expressed in the publication of over one hundred scientific papers and summarized in his editorship of the sixth, seventh, and eighth editions of Gradwohl's Clinical Laboratory Methods and Diagnosis (1963, 1970, 1980). He and his colleagues of the Microbiology Laboratory at Jewish Hospital were leading evaluators of new equipment and procedures, particularly of automated testing instrumentation.
Sonnenwirth was for many years a key participant in professional associations of microbiologists and their conferences, symposiums, and seminars. This activity included extensive travel within the U.S. and abroad. Sonnenwirth is remembered for his services to the American Society for Microbiology, having been among the organizers of the Clinical Microbiology Section in 1963 and its chairman from 1970 to 1973. Sonnenwirth was chosen by the American Society for Microbiology to receive its highest professional recognition, the Becton-Dickinson Award, in 1984.