Santi Sergio e Bacco al Foro Romano was a small titular church, now demolished, attached to the south side of the Arch of Septimus Severus in the Roman Forum. Picture of church on Wikimedia Commons.  It features on an English Wikipedia page. 
The first unambiguous reference to the church in the Liber Pontificalis is to the reign of Pope Gregory III (731-41). He is credited with the building of the church on the ruins of an ancient oratory, the seat of a diaconia. It has been surmised that the original diaconia or centre for Church charitable activities was located in the former Temple of Concord before that building collapsed, but the documentary evidence does not support this.
The interesting thing about this little 8th century church is that it was constructed on new foundations next to the Arch of Septimus Severus. This hints that this end of the Roman Forum may already have suffered dereliction, perhaps as a result of earthquake and a landslide from the Capitoline Hill. However, the archaelogical evidence that would have elucidated this point was destroyed by slapdash excavation without stratigraphy in the 19th century. Other indications are that the Forum was continuing at least in part to function as a civic space until the 11th century.
Before the Sack of Rome by the armies of the Holy Roman Emperor Charles V in 1527, the Forum was a populated neighbourhood and not the field of ruins that it later was. The occupation debris partly buried the surviving ancient monuments in the course of time, including the Arch of Septimus Severus, in a way familiar in later engravings.
The oratory was rebuilt as a proper church with larger dimensions in the reign of Pope Adrian I (772-95). It was rebuilt again, very likely at a higher ground level, in the reign of Pope Innocent III(1198-1216) and was provided with a portico supported by "many" columns. There was a further restoration in the reign of Pope Sixtus IV (1471-84) after the church had fallen into disrepair.
The first cardinal deacon was appointed to the church allegedly in the reign of Pope Agatho (678-81), but he is not in the lists if so. The first on record was Abbot Desiderius of Montecassino in 1058, a Benedictine monk who later became Pope Victor III. This hints that there may have been a monastery attached to the church at that time. The last cardinal died in 1559, and the title was then vacant until it was suppressed in 1589.
The English Wikipedia page referred to has a list of the cardinals.
Relationship to the archEdit
It is on record that the church was granted possession of half of the Arch of Septimus Severus in 1195. The arch was hence shared between two users. The northern pier and the rooms in the top belonged to a local noble family, who treated it as a fortress and built a look-out turret on the north end. (This features in late 16th century engravings, but had gone by the time Piranesi published his drawings.) The southern pier was part of the church, which connected with the void within by means of a doorway and which allegedly had a short campanile on top of the arch. This published allegation seems to be misidentifying the turret mentioned.
A drawing published by Marten Heenskerk in 1536 shows the church, just after it had been demolished. The view is from the apse end, and shows a small edifice on a rectangular plan, without aisles and with a an external segmental apse. There are two round-headed windows in the side wall on view, and the merest hint of an entrance portico. Most importantly, a Romanesque campanile is attached to the left hand side of the façade (hence, not on top of the arch), with two storeys above the side roofline and a tiled pyramidal cap. The side of the top storey visible had an arcade of three arches.
In 1534 Pope Paul III created a new road from the Campodoglio, running through the Arch of Septimus Severus and the Forum to the present Arch of Titus, which then was a fortified gateway to the domain of the Frangipani family on the Palatine. This was for a solemn procession of the emperor on a state visit, and entailed the demolition of two hundred houses and allegedly four churches. Santi Sergio e Bacco was one of these. An avenue of elm trees was planted, and the open area created became the setting for the cattle market which struck the sensibilities of romantics in later centuries. The existence of the Roman Forum as a populated mediaeval neighbourhood was almost forgotten, and even now is very badly documented.
Archaeological excavations in 1812 revealed the foundations of the church and much of the apse walling, but these were not properly recorded before being destroyed in order to reveal the ancient Roman ground surface beneath.
The church had been the headquarters of the guild of butchers, who were given the church of Santa Maria della Quercia after the demolition.
Other churches in the Forum lost in 1534Edit
The other three demolished churches were apparently:
Santi Pietro e Paolo, in the Basilica of Maxentius, built in the 760's.
San Giovanni, in the Basilica Aemilia, 8th century.
Santa Maria in Cannapara, in the Basilica Julia. 8th or 9th century. This was renamed Santa Maria delle Grazie in the 15th century.
Watkin, D: The Roman Forum, Profile Books 2009
Armellini, M: La Chiese di Roma, 1891, p538.
Ridley, R: The Eagle and the Spade, Archaeology in Rome during the Napoleonic Era. CUP 1992